diff --git a/lib/datasets/house-of-representative.rb b/lib/datasets/house-of-representative.rb
index 8ccdc14e..e42c652b 100644
--- a/lib/datasets/house-of-representative.rb
+++ b/lib/datasets/house-of-representative.rb
@@ -76,10 +76,11 @@ def open_data
       data_path = cache_dir_path + "gian.csv"
       download(data_path, data_url)
 
+      parser = JapaneseDateParser.new
       japanese_date_converter = lambda do |field, info|
         case info.header
         when /年月日\z/
-          JapaneseDateParser.new(field).parse
+          parser.parse(field)
         else
           field
         end
diff --git a/lib/datasets/japanese-date-parser.rb b/lib/datasets/japanese-date-parser.rb
index 46b27963..2d596e4b 100644
--- a/lib/datasets/japanese-date-parser.rb
+++ b/lib/datasets/japanese-date-parser.rb
@@ -7,12 +7,8 @@ class UnsupportedEraInitialRange < Error; end
       "令和" => "R",
     }.freeze
 
-    def initialize(string)
-      @string = string
-    end
-
-    def parse
-      case @string
+    def parse(string)
+      case string
       when nil
         nil
       when /\A(平成|令和|..)\s*(\d{1,2}|元)年\s*(\d{1,2})月\s*(\d{1,2})日\z/
@@ -35,7 +31,7 @@ def parse
         day = match_data[4].rjust(2, "0")
         Date.jisx0301("#{era_initial}#{year}.#{month}.#{day}")
       else
-        @string
+        string
       end
     end
   end
diff --git a/test/japanese-date-parser-test.rb b/test/japanese-date-parser-test.rb
index 88b062b4..7004db16 100644
--- a/test/japanese-date-parser-test.rb
+++ b/test/japanese-date-parser-test.rb
@@ -1,4 +1,8 @@
 class JapaneseDateParserTest < Test::Unit::TestCase
+  def setup
+    @parser = Datasets::JapaneseDateParser.new
+  end
+
   data("month and day with leading a space in Heisei", ["H10.01.01", "平成10年 1月 1日"])
   data("month         with leading a space in Heisei", ["H10.01.10", "平成10年 1月10日"])
   data("          day with leading a space in Heisei", ["H10.10.01", "平成10年10月 1日"])
@@ -11,13 +15,13 @@ class JapaneseDateParserTest < Test::Unit::TestCase
   data("boundary within Reiwa", ["R01.05.01", "令和元年 5月 1日"])
   test("#parse") do
     expected_jisx0301, japanese_date_string = data
-    assert_equal(expected_jisx0301, Datasets::JapaneseDateParser.new(japanese_date_string).parse.jisx0301)
+    assert_equal(expected_jisx0301, @parser.parse(japanese_date_string).jisx0301)
   end
 
   test("unsupported era initial range") do
     expected_message = "era must be one of [平成, 令和]: 昭和"
     assert_raise(Datasets::JapaneseDateParser::UnsupportedEraInitialRange.new(expected_message)) do
-      Datasets::JapaneseDateParser.new("昭和元年 1月 1日").parse
+      @parser.parse(("昭和元年 1月 1日"))
     end
   end
 end