diff --git a/backend/app/services/post_tag_search.rb b/backend/app/services/post_tag_search.rb
index 461400d..0eed657 100644
--- a/backend/app/services/post_tag_search.rb
+++ b/backend/app/services/post_tag_search.rb
@@ -4,11 +4,11 @@ class PostTagSearch
     tokens = raw.split.reject { _1.downcase == 'type:or' }
     names = tokens.map { _1.sub(/\Anot:/i, '') }
     registered = TagName.where(language_code: locale.language_code, name: names)
-        .pluck(:name, :tag_id).to_h
+        .pluck(:name, :tag_id).to_h.transform_keys(&:downcase)
     primary = TagName.where(language_code: locale.language_code,
         tag_id: registered.values, primary_flg: true).pluck(:tag_id, :name).to_h
     tokens.zip(names).map do |token, name|
-      { name: primary[registered[name]] || name,
+      { name: primary[registered[name.downcase]] || name,
         negative: token.downcase.start_with?('not:') }
     end
   end
@@ -20,11 +20,11 @@ class PostTagSearch
   def self.references_for(locale, literals)
     names = literals.map { _1[:name] }.uniq
     internal = TagName.where(language_code: locale.language_code, name: names)
-        .pluck(:name, :tag_id).group_by(&:first)
+        .pluck(:name, :tag_id).group_by { _1.first.downcase }
     external = names.empty? ? {} : ExternalTag.where(
         "CONCAT(platform, ':', name) IN (?)", names)
-        .pluck(Arel.sql("CONCAT(platform, ':', name)"), :id).group_by(&:first)
-    names.to_h { |name| [name, [internal.fetch(name, []).map(&:last).uniq,
-                              external.fetch(name, []).map(&:last).uniq]] }
+        .pluck(Arel.sql("CONCAT(platform, ':', name)"), :id).group_by { _1.first.downcase }
+    names.to_h { |name| [name, [internal.fetch(name.downcase, []).map(&:last).uniq,
+                              external.fetch(name.downcase, []).map(&:last).uniq]] }
   end
 end