diff --git a/lib/net/http/header.rb b/lib/net/http/header.rb
index 6106e794..a39043e8 100644
--- a/lib/net/http/header.rb
+++ b/lib/net/http/header.rb
@@ -631,11 +631,35 @@ def set_range(r, e = nil)
   #   res = Net::HTTP.get_response(hostname, '/todos/1')
   #   res.content_length # => nil
   #
+  # The value must consist of digits only.
+  # Multiple <tt>'Content-Length'</tt> values are accepted
+  # only when they are all identical;
+  # otherwise Net::HTTPHeaderSyntaxError is raised.
+  # See {RFC 9110 Section 8.6}[https://www.rfc-editor.org/rfc/rfc9110.html#section-8.6].
   def content_length
-    return nil unless key?('Content-Length')
-    len = self['Content-Length'].slice(/\d+/) or
-        raise Net::HTTPHeaderSyntaxError, 'wrong Content-Length format'
-    len.to_i
+    values = @header['content-length']
+    return nil if values.nil?
+
+    lengths = []
+    values.each do |value|
+      value = value.strip
+      if value.empty?
+        raise Net::HTTPHeaderSyntaxError,
+              "empty Content-Length value"
+      end
+      lengths.concat(value.split(/\s*,\s*/, -1))
+    end
+    if lengths.uniq.size != 1
+      raise Net::HTTPHeaderSyntaxError,
+            "Content-Length has multiple different values: " +
+            values.join(", ")
+    end
+    length = lengths.first
+    unless /\A\d+\z/.match?(length)
+      raise Net::HTTPHeaderSyntaxError,
+            "wrong Content-Length format: #{length}"
+    end
+    length.to_i
   end
 
   # Sets the value of field <tt>'Content-Length'</tt> to the given numeric;
diff --git a/test/net/http/test_httpheader.rb b/test/net/http/test_httpheader.rb
index f4b78603..3c09f4c1 100644
--- a/test/net/http/test_httpheader.rb
+++ b/test/net/http/test_httpheader.rb
@@ -396,12 +396,17 @@ def test_content_length
     try_content_length 500, '500'
     try_content_length 10000_0000_0000, '1000000000000'
     try_content_length 123, '  123'
-    try_content_length 1,   '1 23'
-    try_content_length 500, '(OK)500'
-    assert_raise(Net::HTTPHeaderSyntaxError, 'here is no digit, but') {
-      @c['content-length'] = 'no digit'
-      @c.content_length
-    }
+
+    # Same values in one Content-Length field are accepted.
+    # See: https://www.rfc-editor.org/rfc/rfc9110.html#section-8.6-13
+    try_content_length 5, '5, 5'
+
+    # Same values in multiple Content-Length fields are accepted.
+    # See: https://www.rfc-editor.org/rfc/rfc9110.html#section-8.6-13
+    @c.delete('content-length')
+    @c.add_field('content-length', '7')
+    @c.add_field('content-length', '7')
+    assert_equal 7, @c.content_length
   end
 
   def try_content_length(len, str)
@@ -409,6 +414,37 @@ def try_content_length(len, str)
     assert_equal len, @c.content_length
   end
 
+  def test_content_length_invalid
+    try_invalid_content_length ''
+    try_invalid_content_length '1 23'
+    try_invalid_content_length '(OK)500'
+    try_invalid_content_length 'no digit'
+    try_invalid_content_length 'abc5'
+    try_invalid_content_length '5abc'
+    try_invalid_content_length '5, 6'
+
+    @c.delete('content-length')
+    @c.add_field('content-length', '7')
+    @c.add_field('content-length', '8')
+    assert_raise(Net::HTTPHeaderSyntaxError) {
+      @c.content_length
+    }
+
+    @c.delete('content-length')
+    @c.add_field('content-length', '5')
+    @c.add_field('content-length', '')
+    assert_raise(Net::HTTPHeaderSyntaxError) {
+      @c.content_length
+    }
+  end
+
+  def try_invalid_content_length(str)
+    @c['content-length'] = str
+    assert_raise(Net::HTTPHeaderSyntaxError, str) {
+      @c.content_length
+    }
+  end
+
   def test_content_length=
     @c.content_length = 0
     assert_equal 0, @c.content_length
diff --git a/test/net/http/test_httpresponse.rb b/test/net/http/test_httpresponse.rb
index 7e7ae8a8..e6d8f412 100644
--- a/test/net/http/test_httpresponse.rb
+++ b/test/net/http/test_httpresponse.rb
@@ -64,6 +64,64 @@ def test_read_body
 Connection: close
 Content-Length: 5
 
+hello
+EOS
+
+    res = Net::HTTPResponse.read_new(io)
+
+    body = nil
+
+    res.reading_body io, true do
+      body = res.read_body
+    end
+
+    assert_equal 'hello', body
+  end
+
+  def test_read_body_invalid_content_length
+    io = dummy_io(<<EOS)
+HTTP/1.1 200 OK
+Connection: close
+Content-Length: abc5
+
+hello
+EOS
+
+    res = Net::HTTPResponse.read_new(io)
+
+    assert_raise(Net::HTTPHeaderSyntaxError) do
+      res.reading_body io, true do
+        res.read_body
+      end
+    end
+  end
+
+  def test_read_body_multiple_different_content_length
+    io = dummy_io(<<EOS)
+HTTP/1.1 200 OK
+Connection: close
+Content-Length: 5
+Content-Length: 100
+
+hello
+EOS
+
+    res = Net::HTTPResponse.read_new(io)
+
+    assert_raise(Net::HTTPHeaderSyntaxError) do
+      res.reading_body io, true do
+        res.read_body
+      end
+    end
+  end
+
+  def test_read_body_multiple_same_content_length
+    io = dummy_io(<<EOS)
+HTTP/1.1 200 OK
+Connection: close
+Content-Length: 5
+Content-Length: 5
+
 hello
 EOS
 
diff --git a/test/net/http/utils.rb b/test/net/http/utils.rb
index 3618a92b..cdba47d6 100644
--- a/test/net/http/utils.rb
+++ b/test/net/http/utils.rb
@@ -209,9 +209,9 @@ def finish
       def build_response_headers
         response = "HTTP/1.1 #{@status} #{status_message(@status)}\r\n"
         if @chunked
-          @headers['Transfer-Encoding'] = 'chunked'
+          self['Transfer-Encoding'] = 'chunked'
         else
-          @headers['Content-Length'] = @body.bytesize.to_s
+          self['Content-Length'] ||= @body.bytesize.to_s
         end
         @headers.each do |key, value|
           response << "#{key}: #{value}\r\n"