To perform no matching processing of sheets in the case that a subsequent sheet is larger than a preceding sheet.

A sheet processor includes: a tray 129 for mounting a carried sheet thereon; an offset roller 407 for moving a sheet in a width direction each time the sheet is mounted on the tray; a side end stopper 416 for receiving the sheets moved in the width direction by the offset roller to match the end parts of the sheets; a stapler unit 132 for binding the sheets with the end parts matched by the side end stopper when the prescribed number of sheets is mounted on the tray; a buffer unit 140 for storing a carried sheet upstream in a sheet carrying direction while the stapler operates; and a CPU 300 for operating the buffer unit. The CPU stops a buffer operation of the buffer unit in the case that the width size of the carried sheet is larger than the width size of the already stored sheets when making the buffer unit perform the buffer operation.