To smoothly cut a bookbinding sheet bundle, even if a cutting removing width of the bookbinding sheet bundle is a width exceeding a maximum cuttable width.

This sheet cutter H can accurately cut a predetermined position without disturbing an end part of the bookbinding sheet bundle, by cutting a residual part having the maximum cuttable width Lmax or less, when a distance Le of the dividing movement becomes the maximum cuttable width Lmax or less, by cutting the bookbinding sheet bundle by a cutting blade 81 with every dividing movement, and by fractionally moving the bookbinding sheet bundle over a distance corresponding to the cutting removing width L by a first carrier belt 309, a second carrier belt 310, a third carrier belt 320 and a fourth carrier belt 321, when the cutting removing width L of an inputted bookbinding sheet bundle P3 exceeds the maximum cuttable width Lmax.