diff --git a/05.wc/lib/wc_methods.rb b/05.wc/lib/wc_methods.rb
new file mode 100644
index 0000000000..b506ddbf6d
--- /dev/null
+++ b/05.wc/lib/wc_methods.rb
@@ -0,0 +1,76 @@
+# frozen_string_literal: true
+
+require 'optparse'
+require_relative './word_count'
+
+OPTION_STRING = 'lwc'
+
+OPTION_NAME_TO_WORD_COUNT_TYPE = OPTION_STRING.chars.zip(WordCount::TYPES).to_h.freeze
+
+def main(args)
+  displayed_items = parse_args(args)
+
+  wc_paths = args.empty? ? [WordCount::Pathname.new] : args.map { WordCount::Pathname.new(_1) }
+
+  word_count_types = WordCount.extract_types(displayed_items)
+
+  output_format = displayed_output_format(displayed_items, wc_paths)
+
+  results =
+    wc_paths.map { _1.word_count_result(word_count_types) }
+            .each { print_word_count_result(output_format, _1) }
+
+  if wc_paths.size >= 2
+    count_total =
+      word_count_types.to_h { [_1, 0] }
+                      .merge!(*results.filter_map { _1[:count] }) { |_, total, count| total + count }
+
+    print_word_count_result(output_format, { path: 'total', count: count_total, message: nil })
+  end
+
+  results.any? { _1[:message] } ? 1 : 0
+end
+
+def parse_args(args)
+  parsed_options = OptionParser.new.getopts(args, OPTION_STRING).transform_keys(OPTION_NAME_TO_WORD_COUNT_TYPE)
+
+  # `wc [file ...]` == `wc -lwc [file ...]`
+  parsed_options.transform_values! { |_| true } unless parsed_options.value?(true)
+
+  parsed_options[:path] = !args.empty?
+
+  parsed_options.select { |_, val| val }.keys
+end
+
+def displayed_output_format(displayed_items, wc_paths)
+  one_type_one_operand = WordCount.extract_types(displayed_items).size == 1 && wc_paths.size == 1
+
+  digit = one_type_one_operand ? 1 : adjust_digit(wc_paths)
+
+  displayed_items.map { output_format_string(_1, digit) }.join(' ')
+end
+
+def adjust_digit(wc_paths)
+  default_digit = wc_paths.any?(&:exist_non_regular_file?) ? 7 : 1
+
+  total_bytes_digit = wc_paths.sum(&:regular_file_size).to_s.size
+
+  [default_digit, total_bytes_digit].max
+end
+
+def output_format_string(displayed_item, digit)
+  case displayed_item
+  when *WordCount::TYPES
+    "%<#{displayed_item}>#{digit}d"
+  when :path
+    '%<path>s'
+  else
+    raise ArgumentError, "displayed_item: allow only #{[*WordCount::TYPES, :path].map(&:inspect).join(', ')}"
+  end
+end
+
+def print_word_count_result(output_format, result)
+  warn "wc: #{result[:path]}: #{result[:message]}" if result[:message]
+
+  puts format(output_format, **result[:count], path: result[:path]) unless result[:count].nil?
+end
diff --git a/05.wc/lib/word_count.rb b/05.wc/lib/word_count.rb
new file mode 100644
index 0000000000..0d4dc99534
--- /dev/null
+++ b/05.wc/lib/word_count.rb
@@ -0,0 +1,138 @@
+# frozen_string_literal: true
+
+module WordCount
+  TYPES = %i[newline word bytesize].freeze
+
+  def extract_types(types)
+    TYPES & types
+  end
+
+  module_function :extract_types
+end
+
+class WordCount::Pathname
+  USE_FILETEST_MODULE_FUNCTIONS = %i[directory? file? readable? size].freeze
+
+  private_constant :USE_FILETEST_MODULE_FUNCTIONS
+
+  def initialize(path = nil)
+    @path = path.to_s
+  end
+
+  def to_path
+    return '-' if @path.empty?
+
+    @path
+  end
+
+  def stdin?
+    to_path == '-'
+  end
+
+  def inspect
+    "#<#{self.class}:#{to_path}>"
+  end
+
+  def open(mode = 'r', perm = 0o0666, &block)
+    return block&.call($stdin) || $stdin if stdin?
+
+    File.open(to_path, mode, perm, &block)
+  end
+
+  def exist?
+    stdin? || FileTest.exist?(to_path)
+  end
+
+  USE_FILETEST_MODULE_FUNCTIONS.each do |method|
+    define_method(method) { stdin? ? $stdin.stat.public_send(method) : FileTest.public_send(method, to_path) }
+  end
+
+  def regular_file_size
+    file? ? size : 0
+  end
+
+  def exist_non_regular_file?
+    exist? && !file?
+  end
+
+  def word_count_result(word_count_types = WordCount::TYPES)
+    path = @path.empty? ? 'standard input' : @path
+
+    return { path:, count: nil, message: exist? ? 'Permission denied' : 'No such file or directory' } unless readable?
+
+    return { path:, count: word_count_types.to_h { [_1, 0] }, message: 'Is a directory' } if directory?
+
+    { path:, count: word_count(word_count_types), message: nil }
+  rescue Errno::EPERM => e
+    { path:, count: nil, message: e.message.partition(' @ ').first }
+  end
+
+  private
+
+  def word_count(word_count_types)
+    return open { _1.set_encoding('ASCII-8BIT').word_count(word_count_types) } unless file? && word_count_types.include?(:bytesize)
+
+    return { bytesize: size } if word_count_types == %i[bytesize]
+
+    counts = open { _1.set_encoding('ASCII-8BIT').word_count(word_count_types - %i[bytesize]) }
+
+    { **counts, bytesize: size }
+  end
+end
+
+module WordCount::IO
+  BUFFER_SIZE = 16 * 1024
+
+  private_constant :BUFFER_SIZE
+
+  def word_count(word_count_types = WordCount::TYPES, bufsize: BUFFER_SIZE)
+    each_buffer(bufsize).inject(:<<).to_s.word_count(word_count_types)
+  end
+
+  def each_buffer(limit = BUFFER_SIZE)
+    return to_enum(__callee__, limit) unless block_given?
+
+    loop do
+      yield readpartial(limit)
+    rescue EOFError
+      break
+    end
+
+    self
+  end
+end
+
+module WordCount::String
+  def word_count(word_count_types = WordCount::TYPES)
+    word_count_types.to_h do |type|
+      case type
+      when *WordCount::TYPES
+        [type, __send__(type)]
+      else
+        raise ArgumentError, "word_count_type: allow only #{WordCount::TYPES.map(&:inspect).join(', ')}"
+      end
+    end
+  end
+
+  private
+
+  def newline
+    count("\n")
+  end
+
+  def word
+    num = 0
+
+    split { num += 1 if _1.match?(/[[:graph:]]/) }
+
+    num
+  end
+end
+
+class IO
+  include WordCount::IO
+end
+
+class String
+  include WordCount::String
+end
diff --git a/05.wc/wc.rb b/05.wc/wc.rb
new file mode 100755
index 0000000000..e272b0508a
--- /dev/null
+++ b/05.wc/wc.rb
@@ -0,0 +1,8 @@
+#!/usr/bin/env ruby
+# frozen_string_literal: true
+
+require_relative './lib/wc_methods'
+
+errno = main(ARGV)
+
+exit(errno)