Abstract: A sheet processing device includes: a stacking unit configured to stack a plurality of sheets to obtain a sheet bundle; a stitched-sheet number acquiring unit configured to acquire a number of sheets in the sheet bundle, which is a target to be stitched; a permanent stitching unit configured to permanently stitch the sheet bundle; a temporary stitching unit configured to be capable of temporarily stitching a plurality of areas of the sheet bundle; a selecting unit configured to select any one of the permanent stitching unit and the temporary stitching unit to be used; and a stitching-process control unit configured to, when the temporary stitching unit is selected by the selecting unit, control the temporary stitching unit so as to change a number of areas that are stitched by the temporary stitching unit.

Abstract: Reference clock signals of the same frequency as that of image scanning clock signals are applied to a shift register, and corrective clock signals of a higher frequency than that of the reference clock signals C.sub.0 are applied as shift clock signals to the shift register, which generates n output signals C.sub.1, C.sub.2, . . . C.sub.n (n.gtoreq.2) which are applied to delay circuits. The delay circuits produce pulse signal groups which are successively delayed .DELTA.t in phase. Assuming that the frequency of the reference clock signals is f.sub.0 MHz and the frequency of the corrective clock signals is n.multidot.f.sub.0 MHz, the above phase .DELTA.t is selected to meet meets the relationship: ##EQU1## One of the pulse signal groups thus delayed .DELTA.t in phase is selected as image scannnig clock signals in response to an output signal from a light sensor that detects a scanning beam for synchronizing main scanning cycles.