diff --git a/.rubocop_rubyzip.yml b/.rubocop_rubyzip.yml
index 33fa619..58deec6 100644
--- a/.rubocop_rubyzip.yml
+++ b/.rubocop_rubyzip.yml
@@ -134,10 +134,10 @@ Style/EmptyLinesAroundMethodBody:
 Style/FileName:
   Enabled: false
 
-# Offense count: 2
+# Offense count: 1
 # Configuration parameters: EnforcedStyle, SupportedStyles.
 Style/FormatString:
-  Enabled: false
+  Enabled: true
 
 # Offense count: 1
 # Cop supports --auto-correct.
diff --git a/lib/zip/file.rb b/lib/zip/file.rb
index b1286b0..a968062 100755
--- a/lib/zip/file.rb
+++ b/lib/zip/file.rb
@@ -180,7 +180,7 @@ module Zip
       def save_splited_part(zip_file, partial_zip_file_name, zip_file_size, szip_file_index, segment_size, segment_count)
         ssegment_size  = zip_file_size - zip_file.pos
         ssegment_size  = segment_size if ssegment_size > segment_size
-        szip_file_name = "#{partial_zip_file_name}.#{'%03d' % (szip_file_index)}"
+        szip_file_name = "#{partial_zip_file_name}.#{format('%03d', szip_file_index)}"
         ::File.open(szip_file_name, 'wb') do |szip_file|
           if szip_file_index == 1
             ssegment_size = put_split_signature(szip_file, segment_size)