diff --git a/CHANGELOG.md b/CHANGELOG.md
index 65a61437edd4..a17ca4abd936 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -47,6 +47,7 @@
 * [#4047](https://github.com/bbatsov/rubocop/issues/4047): Allow `find_zone` and `find_zone!` methods in `Rails/TimeZone`. ([@attilahorvath][])
 * [#3457](https://github.com/bbatsov/rubocop/issues/3457): Clear a warning and prevent new warnings. ([@mikegee][])
 * [#4066](https://github.com/bbatsov/rubocop/issues/4066): Register an offense in `Lint/ShadowedException` when an exception is shadowed and there is an implicit begin. ([@rrosenblum][])
+* [#4001](https://github.com/bbatsov/rubocop/issues/4001): Lint/UnneededDisable of Metrics/LineLength that isn't unneeded. ([@wkurniawan07][])
 
 ## 0.47.1 (2017-01-18)
 
@@ -2655,3 +2656,4 @@
 [@dorian]: https://github.com/dorian
 [@attilahorvath]: https://github.com/attilahorvath
 [@droptheplot]: https://github.com/droptheplot
+[@wkurniawan07]: https://github.com/wkurniawan07
diff --git a/lib/rubocop/comment_config.rb b/lib/rubocop/comment_config.rb
index caa517822742..dc1fab2325b1 100644
--- a/lib/rubocop/comment_config.rb
+++ b/lib/rubocop/comment_config.rb
@@ -11,7 +11,7 @@ class CommentConfig
     COPS_PATTERN = "(all|#{COP_NAMES_PATTERN})".freeze
 
     COMMENT_DIRECTIVE_REGEXP = Regexp.new(
-      ('\A# rubocop : ((?:dis|en)able)\b ' + COPS_PATTERN).gsub(' ', '\s*')
+      ('# rubocop : ((?:dis|en)able)\b ' + COPS_PATTERN).gsub(' ', '\s*')
     )
 
     CopAnalysis = Struct.new(:line_ranges, :start_line_number)
diff --git a/lib/rubocop/cop/metrics/line_length.rb b/lib/rubocop/cop/metrics/line_length.rb
index b028bc0ac834..6c85ef6a2553 100644
--- a/lib/rubocop/cop/metrics/line_length.rb
+++ b/lib/rubocop/cop/metrics/line_length.rb
@@ -159,7 +159,7 @@ def directive_on_source_line?(index)
         end
 
         def line_length_without_directive(line)
-          before_comment, = line.split('#')
+          before_comment, = line.split(CommentConfig::COMMENT_DIRECTIVE_REGEXP)
           before_comment.rstrip.length
         end
 
diff --git a/spec/rubocop/cop/metrics/line_length_spec.rb b/spec/rubocop/cop/metrics/line_length_spec.rb
index 62fe568512b0..40dbc66ef643 100644
--- a/spec/rubocop/cop/metrics/line_length_spec.rb
+++ b/spec/rubocop/cop/metrics/line_length_spec.rb
@@ -305,6 +305,38 @@ def method_definition_that_is_just_under_the_line_length_limit(foo) # rubocop:di
         inspect_source(cop, source)
         expect(cop.highlights).to eq(['bcd'])
       end
+
+      context 'and the source contains non-directive # as comment' do
+        let(:source) { <<-END }
+          aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa # bbbbbbbbbbbbbb # rubocop:enable Style/ClassVars'
+        END
+
+        it 'registers an offense for the line' do
+          inspect_source(cop, source)
+          expect(cop.offenses.size).to eq(1)
+        end
+
+        it 'highlights only the non-directive part' do
+          inspect_source(cop, source)
+          expect(cop.highlights).to eq(['bbbbbbb'])
+        end
+      end
+
+      context 'and the source contains non-directive #s as non-comment' do
+        let(:source) { <<-END }
+          LARGE_DATA_STRING_PATTERN = %r{\A([A-Za-z0-9\+\/#]*\={0,2})#([A-Za-z0-9\+\/#]*\={0,2})#([A-Za-z0-9\+\/#]*\={0,2})\z} # rubocop:disable LineLength
+        END
+
+        it 'registers an offense for the line' do
+          inspect_source(cop, source)
+          expect(cop.offenses.size).to eq(1)
+        end
+
+        it 'highlights only the non-directive part' do
+          inspect_source(cop, source)
+          expect(cop.highlights).to eq([']*={0,2})#([A-Za-z0-9+/#]*={0,2})z}'])
+        end
+      end
     end
   end
 end