diff --git a/src/main/scala/net/yoshinorin/qualtet/domains/search/SearchService.scala b/src/main/scala/net/yoshinorin/qualtet/domains/search/SearchService.scala
index 34c20bde..5aa82414 100644
--- a/src/main/scala/net/yoshinorin/qualtet/domains/search/SearchService.scala
+++ b/src/main/scala/net/yoshinorin/qualtet/domains/search/SearchService.scala
@@ -7,7 +7,7 @@ import cats.implicits.*
 import net.yoshinorin.qualtet.config.SearchConfig
 import net.yoshinorin.qualtet.infrastructure.db.Executer
 import net.yoshinorin.qualtet.message.Fail.UnprocessableEntity
-import net.yoshinorin.qualtet.http.{Error => Err}
+import net.yoshinorin.qualtet.http.ProblemDetailsError
 import net.yoshinorin.qualtet.types.Points
 import net.yoshinorin.qualtet.syntax.*
 
@@ -27,11 +27,11 @@ class SearchService[F[_]: Monad](
   private[search] def extractQueryStringsFromQuery(query: Map[String, List[String]]): List[String] = query.getOrElse("q", List()).map(_.trim.toLower)
 
   // TODO: move constant values
-  private[search] def accumurateQueryStringsErrors(queryStrings: List[String]): Seq[Err] = {
-    val validator: (Boolean, Err) => Option[Err] = (cond, err) => { if cond then Some(err) else None }
+  private[search] def accumurateQueryStringsErrors(queryStrings: List[String]): Seq[ProblemDetailsError] = {
+    val validator: (Boolean, ProblemDetailsError) => Option[ProblemDetailsError] = (cond, err) => { if cond then Some(err) else None }
     val isEmptyError = validator(
       queryStrings.isEmpty,
-      Err(
+      ProblemDetailsError(
         code = "SEARCH_QUERY_REQUIRED",
         message = "Search query required."
       )
@@ -39,7 +39,7 @@ class SearchService[F[_]: Monad](
 
     val tooManySearchWordsError = validator(
       queryStrings.sizeIs > searchConfig.maxWords,
-      Err(
+      ProblemDetailsError(
         code = "TOO_MANY_SEARCH_WORDS",
         message = s"Search words must be less than ${searchConfig.maxWords}. You specified ${queryStrings.size}."
       )
@@ -49,21 +49,21 @@ class SearchService[F[_]: Monad](
       List(
         validator(
           q.hasIgnoreChars,
-          Err(
+          ProblemDetailsError(
             code = "INVALID_CHARS_INCLUDED",
             message = s"Contains unusable chars in ${q}"
           )
         ),
         validator(
           q.length < searchConfig.minWordLength,
-          Err(
+          ProblemDetailsError(
             code = "SEARCH_CHAR_LENGTH_TOO_SHORT",
             message = s"${q} is too short. You must be more than ${searchConfig.minWordLength} chars in one word."
           )
         ),
         validator(
           q.length > searchConfig.maxWordLength,
-          Err(
+          ProblemDetailsError(
             code = "SEARCH_CHAR_LENGTH_TOO_LONG",
             message = s"${q} is too long. You must be less than ${searchConfig.maxWordLength} chars in one word."
           )
diff --git a/src/main/scala/net/yoshinorin/qualtet/http/ResponseProblemDetails.scala b/src/main/scala/net/yoshinorin/qualtet/http/ResponseProblemDetails.scala
index 2ed8d7b7..e9d8f081 100644
--- a/src/main/scala/net/yoshinorin/qualtet/http/ResponseProblemDetails.scala
+++ b/src/main/scala/net/yoshinorin/qualtet/http/ResponseProblemDetails.scala
@@ -10,19 +10,19 @@ final case class ResponseProblemDetails(
   status: Int,
   detail: String,
   instance: String,
-  errors: Option[Seq[Error]] = None
+  errors: Option[Seq[ProblemDetailsError]] = None
 )
 
 object ResponseProblemDetails {
   given codecResponseProblemDetails: JsonValueCodec[ResponseProblemDetails] = JsonCodecMaker.make
 }
 
-final case class Error(
+final case class ProblemDetailsError(
   code: String,
   message: String
 )
 
-object Error {
-  given codecError: JsonValueCodec[Error] = JsonCodecMaker.make
-  given codecErrors: JsonValueCodec[Option[Seq[Error]]] = JsonCodecMaker.make
+object ProblemDetailsError {
+  given codecProblemDetailsError: JsonValueCodec[ProblemDetailsError] = JsonCodecMaker.make
+  given codecProblemDetailsErrors: JsonValueCodec[Option[Seq[ProblemDetailsError]]] = JsonCodecMaker.make
 }
diff --git a/src/main/scala/net/yoshinorin/qualtet/message/Fail.scala b/src/main/scala/net/yoshinorin/qualtet/message/Fail.scala
index 2de6cc36..56823bea 100644
--- a/src/main/scala/net/yoshinorin/qualtet/message/Fail.scala
+++ b/src/main/scala/net/yoshinorin/qualtet/message/Fail.scala
@@ -1,6 +1,6 @@
 package net.yoshinorin.qualtet.message
 
-import net.yoshinorin.qualtet.http.{Error => Err}
+import net.yoshinorin.qualtet.http.ProblemDetailsError
 
 sealed trait Fail extends Exception
 
@@ -9,31 +9,31 @@ object Fail {
   final case class NotFound(
     title: String = "Not Found",
     detail: String,
-    errors: Option[Seq[Err]] = None
+    errors: Option[Seq[ProblemDetailsError]] = None
   ) extends Fail
 
   final case class Unauthorized(
     title: String = "Unauthorized",
     detail: String = "Unauthorized", // NOTE: no-need to set error details for secure reason. set default value instead.
-    errors: Option[Seq[Err]] = None
+    errors: Option[Seq[ProblemDetailsError]] = None
   ) extends Fail
 
   final case class UnprocessableEntity(
     title: String = "Unprocessable Entity",
     detail: String,
-    errors: Option[Seq[Err]] = None
+    errors: Option[Seq[ProblemDetailsError]] = None
   ) extends Fail
 
   final case class BadRequest(
     title: String = "Bad Request",
     detail: String,
-    errors: Option[Seq[Err]] = None
+    errors: Option[Seq[ProblemDetailsError]] = None
   ) extends Fail
 
   final case class Forbidden(
     title: String = "Forbidden",
     detail: String,
-    errors: Option[Seq[Err]] = None
+    errors: Option[Seq[ProblemDetailsError]] = None
   ) extends Fail
 
   final case class InternalServerError(
diff --git a/src/test/scala/net/yoshinorin/qualtet/domains/search/SearchServiceSpec.scala b/src/test/scala/net/yoshinorin/qualtet/domains/search/SearchServiceSpec.scala
index d77cb80c..6b12f6b6 100644
--- a/src/test/scala/net/yoshinorin/qualtet/domains/search/SearchServiceSpec.scala
+++ b/src/test/scala/net/yoshinorin/qualtet/domains/search/SearchServiceSpec.scala
@@ -4,7 +4,7 @@ import net.yoshinorin.qualtet.domains.contents.{Path, RequestContent}
 import net.yoshinorin.qualtet.domains.robots.Attributes
 import net.yoshinorin.qualtet.fixture.Fixture.*
 import net.yoshinorin.qualtet.Modules.*
-import net.yoshinorin.qualtet.http.{Error => Err}
+import net.yoshinorin.qualtet.http.ProblemDetailsError
 import org.scalatest.wordspec.AnyWordSpec
 import org.scalatest.BeforeAndAfterAll
 import net.yoshinorin.qualtet.message.Fail.UnprocessableEntity
@@ -100,7 +100,7 @@ class SearchServiceSpec extends AnyWordSpec with BeforeAndAfterAll {
         )
       )
       assert(
-        result === List(Err(message = "Search query required.", code = "SEARCH_QUERY_REQUIRED"))
+        result === List(ProblemDetailsError(message = "Search query required.", code = "SEARCH_QUERY_REQUIRED"))
       )
     }
 
@@ -118,13 +118,13 @@ class SearchServiceSpec extends AnyWordSpec with BeforeAndAfterAll {
       )
       assert(
         result === List(
-          Err(message = "a is too short. You must be more than 4 chars in one word.", code = "SEARCH_CHAR_LENGTH_TOO_SHORT"),
-          Err(message = "abc is too short. You must be more than 4 chars in one word.", code = "SEARCH_CHAR_LENGTH_TOO_SHORT"),
-          Err(message = "Contains unusable chars in d.", code = "INVALID_CHARS_INCLUDED"),
-          Err(message = "d. is too short. You must be more than 4 chars in one word.", code = "SEARCH_CHAR_LENGTH_TOO_SHORT"),
-          Err(message = "Contains unusable chars in 1234567890123456", code = "INVALID_CHARS_INCLUDED"),
-          Err(message = "1234567890123456 is too long. You must be less than 15 chars in one word.", code = "SEARCH_CHAR_LENGTH_TOO_LONG"),
-          Err(message = "Search words must be less than 3. You specified 7.", code = "TOO_MANY_SEARCH_WORDS")
+          ProblemDetailsError(message = "a is too short. You must be more than 4 chars in one word.", code = "SEARCH_CHAR_LENGTH_TOO_SHORT"),
+          ProblemDetailsError(message = "abc is too short. You must be more than 4 chars in one word.", code = "SEARCH_CHAR_LENGTH_TOO_SHORT"),
+          ProblemDetailsError(message = "Contains unusable chars in d.", code = "INVALID_CHARS_INCLUDED"),
+          ProblemDetailsError(message = "d. is too short. You must be more than 4 chars in one word.", code = "SEARCH_CHAR_LENGTH_TOO_SHORT"),
+          ProblemDetailsError(message = "Contains unusable chars in 1234567890123456", code = "INVALID_CHARS_INCLUDED"),
+          ProblemDetailsError(message = "1234567890123456 is too long. You must be less than 15 chars in one word.", code = "SEARCH_CHAR_LENGTH_TOO_LONG"),
+          ProblemDetailsError(message = "Search words must be less than 3. You specified 7.", code = "TOO_MANY_SEARCH_WORDS")
         )
       )
     }