diff --git a/stdlib/kconv/0/kconv.rbs b/stdlib/kconv/0/kconv.rbs
new file mode 100644
index 000000000..7e2461c86
--- /dev/null
+++ b/stdlib/kconv/0/kconv.rbs
@@ -0,0 +1,166 @@
+# <!-- rdoc-file=ext/nkf/lib/kconv.rb -->
+# Kanji Converter for Ruby.
+#
+module Kconv
+  # <!-- rdoc-file=ext/nkf/lib/kconv.rb -->
+  # ASCII
+  #
+  ASCII: Encoding
+
+  # <!-- rdoc-file=ext/nkf/lib/kconv.rb -->
+  # Auto-Detect
+  #
+  AUTO: nil
+
+  # <!-- rdoc-file=ext/nkf/lib/kconv.rb -->
+  # BINARY
+  #
+  BINARY: Encoding
+
+  # <!-- rdoc-file=ext/nkf/lib/kconv.rb -->
+  # EUC-JP
+  #
+  EUC: Encoding
+
+  # <!-- rdoc-file=ext/nkf/lib/kconv.rb -->
+  # ISO-2022-JP
+  #
+  JIS: Encoding
+
+  # <!-- rdoc-file=ext/nkf/lib/kconv.rb -->
+  # NOCONV
+  #
+  NOCONV: nil
+
+  # <!-- rdoc-file=ext/nkf/lib/kconv.rb -->
+  # Shift_JIS
+  #
+  SJIS: Encoding
+
+  # <!-- rdoc-file=ext/nkf/lib/kconv.rb -->
+  # UNKNOWN
+  #
+  UNKNOWN: nil
+
+  # <!-- rdoc-file=ext/nkf/lib/kconv.rb -->
+  # UTF-16
+  #
+  UTF16: Encoding
+
+  # <!-- rdoc-file=ext/nkf/lib/kconv.rb -->
+  # UTF-32
+  #
+  UTF32: Encoding
+
+  # <!-- rdoc-file=ext/nkf/lib/kconv.rb -->
+  # UTF-8
+  #
+  UTF8: Encoding
+
+  # <!--
+  #   rdoc-file=ext/nkf/lib/kconv.rb
+  #   - Kconv.guess(str)   => encoding
+  # -->
+  # Guess input encoding by NKF.guess
+  #
+  def self.guess: (String str) -> Encoding
+
+  # <!--
+  #   rdoc-file=ext/nkf/lib/kconv.rb
+  #   - Kconv.iseuc(str)   => true or false
+  # -->
+  # Returns whether input encoding is EUC-JP or not.
+  #
+  # **Note** don't expect this return value is MatchData.
+  #
+  def self.iseuc: (String str) -> bool
+
+  # <!--
+  #   rdoc-file=ext/nkf/lib/kconv.rb
+  #   - Kconv.isjis(str)   => true or false
+  # -->
+  # Returns whether input encoding is ISO-2022-JP or not.
+  #
+  def self.isjis: (String str) -> bool
+
+  # <!--
+  #   rdoc-file=ext/nkf/lib/kconv.rb
+  #   - Kconv.issjis(str)   => true or false
+  # -->
+  # Returns whether input encoding is Shift_JIS or not.
+  #
+  def self.issjis: (String str) -> bool
+
+  # <!--
+  #   rdoc-file=ext/nkf/lib/kconv.rb
+  #   - Kconv.isutf8(str)   => true or false
+  # -->
+  # Returns whether input encoding is UTF-8 or not.
+  #
+  def self.isutf8: (String str) -> bool
+
+  # <!--
+  #   rdoc-file=ext/nkf/lib/kconv.rb
+  #   - Kconv.kconv(str, to_enc, from_enc=nil)
+  # -->
+  # Convert `str` to `to_enc`. `to_enc` and `from_enc` are given as constants of
+  # Kconv or Encoding objects.
+  #
+  def self.kconv: (String str, Encoding? out_code, ?Encoding? in_code) -> String
+
+  # <!--
+  #   rdoc-file=ext/nkf/lib/kconv.rb
+  #   - Kconv.toeuc(str)   => string
+  # -->
+  # Convert `str` to EUC-JP
+  #
+  def self.toeuc: (String str) -> String
+
+  # <!--
+  #   rdoc-file=ext/nkf/lib/kconv.rb
+  #   - Kconv.tojis(str)   => string
+  # -->
+  # Convert `str` to ISO-2022-JP
+  #
+  def self.tojis: (String str) -> String
+
+  # <!--
+  #   rdoc-file=ext/nkf/lib/kconv.rb
+  #   - Kconv.tolocale   => string
+  # -->
+  # Convert `self` to locale encoding
+  #
+  def self.tolocale: (String str) -> String
+
+  # <!--
+  #   rdoc-file=ext/nkf/lib/kconv.rb
+  #   - Kconv.tosjis(str)   => string
+  # -->
+  # Convert `str` to Shift_JIS
+  #
+  def self.tosjis: (String str) -> String
+
+  # <!--
+  #   rdoc-file=ext/nkf/lib/kconv.rb
+  #   - Kconv.toutf16(str)   => string
+  # -->
+  # Convert `str` to UTF-16
+  #
+  def self.toutf16: (String str) -> String
+
+  # <!--
+  #   rdoc-file=ext/nkf/lib/kconv.rb
+  #   - Kconv.toutf32(str)   => string
+  # -->
+  # Convert `str` to UTF-32
+  #
+  def self.toutf32: (String str) -> String
+
+  # <!--
+  #   rdoc-file=ext/nkf/lib/kconv.rb
+  #   - Kconv.toutf8(str)   => string
+  # -->
+  # Convert `str` to UTF-8
+  #
+  def self.toutf8: (String str) -> String
+end
diff --git a/test/stdlib/Kconv_test.rb b/test/stdlib/Kconv_test.rb
new file mode 100644
index 000000000..4cac08c3e
--- /dev/null
+++ b/test/stdlib/Kconv_test.rb
@@ -0,0 +1,118 @@
+require_relative "test_helper"
+require "kconv"
+
+class KconvSingletonTest < Test::Unit::TestCase
+  include TestHelper
+
+  library "kconv"
+  testing "singleton(::Kconv)"
+
+  def test_ASCII
+    assert_const_type "::Encoding", "Kconv::ASCII"
+  end
+
+  def test_AUTO
+    assert_const_type "nil", "Kconv::AUTO"
+  end
+
+  def test_BINARY
+    assert_const_type "::Encoding", "Kconv::BINARY"
+  end
+
+  def test_EUC
+    assert_const_type "::Encoding", "Kconv::EUC"
+  end
+
+  def test_JIS
+    assert_const_type "::Encoding", "Kconv::JIS"
+  end
+
+  def test_NOCONV
+    assert_const_type "nil", "Kconv::NOCONV"
+  end
+
+  def test_SJIS
+    assert_const_type "::Encoding", "Kconv::SJIS"
+  end
+
+  def test_UNKNOWN
+    assert_const_type "nil", "Kconv::UNKNOWN"
+  end
+
+  def test_UTF16
+    assert_const_type "::Encoding", "Kconv::UTF16"
+  end
+
+  def test_UTF32
+    assert_const_type "::Encoding", "Kconv::UTF32"
+  end
+
+  def test_UTF8
+    assert_const_type "::Encoding", "Kconv::UTF8"
+  end
+
+  def test_guess
+    assert_send_type "(::String str) -> ::Encoding?",
+                     Kconv, :guess, ""
+  end
+
+  def test_iseuc
+    assert_send_type "(::String str) -> bool",
+                     Kconv, :iseuc, ""
+  end
+
+  def test_isjis
+    assert_send_type "(::String str) -> bool",
+                     Kconv, :isjis, ""
+  end
+
+  def test_issjis
+    assert_send_type "(::String str) -> bool",
+                     Kconv, :issjis, ""
+  end
+
+  def test_isutf8
+    assert_send_type "(::String str) -> bool",
+                     Kconv, :isutf8, ""
+  end
+
+  def test_kconv
+    assert_send_type "(::String str, ::Encoding? out_code, ?::Encoding? in_code) -> ::String",
+                     Kconv, :kconv, "", Kconv::UTF8
+  end
+
+  def test_toeuc
+    assert_send_type "(::String str) -> ::String",
+                     Kconv, :toeuc, ""
+  end
+
+  def test_tojis
+    assert_send_type "(::String str) -> ::String",
+                     Kconv, :tojis, ""
+  end
+
+  def test_tolocale
+    assert_send_type "(::String str) -> ::String",
+                     Kconv, :tolocale, ""
+  end
+
+  def test_tosjis
+    assert_send_type "(::String str) -> ::String",
+                     Kconv, :tosjis, ""
+  end
+
+  def test_toutf16
+    assert_send_type "(::String str) -> ::String",
+                     Kconv, :toutf16, ""
+  end
+
+  def test_toutf32
+    assert_send_type "(::String str) -> ::String",
+                     Kconv, :toutf32, ""
+  end
+
+  def test_toutf8
+    assert_send_type "(::String str) -> ::String",
+                     Kconv, :toutf8, ""
+  end
+end