An apparatus for correcting a bar width comprises a detecting section calculating a reference bar width serving as a reference of bar widths, a first calculation section calculating an average value of a plurality of reference bar widths, a second calculation section calculating an error value between a bar width of a bar to be corrected and the reference bar width calculated by the detecting section, and a correcting section correcting the bar width of the bar to be corrected by using the calculated average value when the calculated error value is not less than a predetermined value.