diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index 4b6a1d6..bea7cac 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -11,7 +11,7 @@ jobs:
   ci:
     runs-on: ubuntu-latest
     container:
-      image: shiguredo/shiguredo-erlang:otp-26.0.2-openssl-3.1.2-ubuntu-22.04-x86_64
+      image: shiguredo/erlang:otp-27.0.1-openssl-3.3.1-ubuntu-24.04-x86_64
     steps:
       - uses: actions/checkout@v4
       - run: make
diff --git a/CHANGES.md b/CHANGES.md
index d4ef730..0606bcb 100644
--- a/CHANGES.md
+++ b/CHANGES.md
@@ -2,9 +2,13 @@
 
 ## develop
 
-- [UPDATE] rebar3 3.22.1 に更新する
+- [CHANGE] rebar3 の minimum_otp_vsn を 27.0 にする
   - @voluntas
-- [UPDATE] GitHub Actions の docker の OTP を 26.0.2 / OpenSSL 3.1.2 に上げる
+- [UPDATE] rebar3 3.23.0 に更新する
+  - @voluntas
+- [UPDATE] GitHub Actions の docker の OTP を 27.0.1 / OpenSSL 3.3.1 に上げる
+  - @voluntas
+- [FIX] import を利用しないようにする
   - @voluntas
 
 ## 2023.1.0
diff --git a/README.md b/README.md
index 40408ad..3cbc10c 100644
--- a/README.md
+++ b/README.md
@@ -8,15 +8,15 @@ Erlang 向けの Base32 ライブラリです。次の種類のバージョン
 
 ## ビルド
 
-```shell
-$ rebar3 compile
+```bash
+rebar3 compile
 ```
 
 ## 利用
 
 例:
 
-```
+```bash
 $ rebar3 shell
 1> base32:decode(clockwork, <<"AXQQEB10D5T20WK5C5P6RY90EXQQ4TVK44">>).
 <<"Wow, it really works!">>
@@ -39,8 +39,8 @@ $ rebar3 shell
 
 ## ライセンス
 
-```
-Copyright 2021-2023, Shiguredo Inc.
+```text
+Copyright 2021-2024, Shiguredo Inc.
 Copyright 2021, SUZUKI Tetsuya (Original Author)
 
 Licensed under the Apache License, Version 2.0 (the "License");
diff --git a/rebar.config b/rebar.config
index 20fee96..745f289 100644
--- a/rebar.config
+++ b/rebar.config
@@ -1,7 +1,8 @@
-{minimum_otp_vsn, "26.0"}.
+{minimum_otp_vsn, "27.0"}.
 
 {erl_opts, [{i, "src"},
             warnings_as_errors,
+            warn_shadow_vars,
             warn_export_all,
             warn_unused_import]}.
 
@@ -12,13 +13,13 @@
                deprecated_functions]}.
 
 %% https://rebar3.org/docs/configuration/configuration/#dialyzer
-{dialyzer, [{warnings, [extra_return,
+{dialyzer, [{warnings, [error_handling,
+                        extra_return,
                         missing_return,
-                        no_unknown
-                        %% overspecs
                         %% underspecs,
+                        unmatched_returns
+                        %% overspecs
                         %% specdiffs
-                        %% error_handling
                        ]},
             incremental,
             {plt_apps, top_level_deps},
diff --git a/rebar3 b/rebar3
index 8cab495..aaeb4cf 100755
Binary files a/rebar3 and b/rebar3 differ
diff --git a/src/base32_clockwork.erl b/src/base32_clockwork.erl
index a48c1e1..99fb4c9 100644
--- a/src/base32_clockwork.erl
+++ b/src/base32_clockwork.erl
@@ -2,8 +2,6 @@
 
 -export([encode/1, decode/1]).
 
--import(base32_utils, [rev_bits_list_to_binary/1, bits_list_size/1]).
-
 
 -spec encode(binary()) -> binary().
 encode(Data) ->
@@ -58,7 +56,7 @@ symbol(30) -> $Y;
 symbol(31) -> $Z.
 
 
--spec decode(binary()) -> {ok, binary()} | {error, atom()}.
+-spec decode(binary()) -> {ok, binary()} | {error, invalid_size | invalid_format}.
 decode(Data) ->
     decode0(Data, []).
 
@@ -66,7 +64,7 @@ decode(Data) ->
 decode0(<<>>, []) ->
     {ok, <<>>};
 decode0(<<>>, Accu = [Last | Prev]) ->
-    Size = bits_list_size(Accu),
+    Size = base32_utils:bits_list_size(Accu),
     Last1 = case Size rem 8 of
                 0 ->
                     Last;
@@ -75,7 +73,7 @@ decode0(<<>>, Accu = [Last | Prev]) ->
                     <<Last2:BodySize/bitstring, _/bitstring>> = Last,
                     Last2
             end,
-    {ok, rev_bits_list_to_binary([Last1 | Prev])};
+    {ok, base32_utils:rev_bits_list_to_binary([Last1 | Prev])};
 decode0(<<_:8>>, []) ->
     {error, invalid_size};
 decode0(<<"0", Next/binary>>, Accu) ->
diff --git a/src/base32_crockford.erl b/src/base32_crockford.erl
index edde797..91a0646 100644
--- a/src/base32_crockford.erl
+++ b/src/base32_crockford.erl
@@ -2,8 +2,6 @@
 
 -export([encode/1, encode_check/1, decode/1, decode_check/1]).
 
--import(base32_utils, [rev_bits_list_to_binary/1]).
-
 
 data_to_integer(Data, Padding) ->
     data_to_integer0(Data, Padding, 0).
@@ -121,7 +119,7 @@ decode(Data) when is_binary(Data) ->
     decode0(Data, []).
 
 
--spec decode_check(binary()) -> {ok, binary()} | {error, atom()}.
+-spec decode_check(binary()) -> {ok, binary()} | {error, invalid}.
 decode_check(Data) ->
     Size = (size(Data) - 1) * 8,
     <<Data0:Size/bitstring, Expectedcheck:8>> = Data,
@@ -134,7 +132,7 @@ decode_check(Data) ->
 
 
 decode0(<<>>, Accu) ->
-    Decoded0 = rev_bits_list_to_binary(Accu),
+    Decoded0 = base32_utils:rev_bits_list_to_binary(Accu),
     DecodedSize = bit_size(Decoded0),
     case DecodedSize rem 8 of
         0 ->
diff --git a/src/base32_rfc4648.erl b/src/base32_rfc4648.erl
index 8d778fd..cd5fba9 100644
--- a/src/base32_rfc4648.erl
+++ b/src/base32_rfc4648.erl
@@ -2,8 +2,6 @@
 
 -export([encode/1, decode/1]).
 
--import(base32_utils, [rev_bits_list_to_binary/1]).
-
 
 -spec encode(binary()) -> binary().
 encode(Data) ->
@@ -160,13 +158,13 @@ encode1(<<1:1>>, Accu) ->
     [$Q | Accu].
 
 
--spec decode(binary()) -> {ok, binary()} | {error, atom()}.
+-spec decode(binary()) -> {ok, binary()} | {error, invalid_format}.
 decode(Data) ->
     decode0(Data, []).
 
 
 decode0(<<>>, Accu) ->
-    {ok, rev_bits_list_to_binary(Accu)};
+    {ok, base32_utils:rev_bits_list_to_binary(Accu)};
 decode0(<<"A", Next/bitstring>>, Accu) ->
     decode0(Next, [<<0:5>> | Accu]);
 decode0(<<"B", Next/bitstring>>, Accu) ->