To provide a sheet processing device which hardly deteriorates an alignment state, even when a sheet where a color image is formed and a sheet where a monochrome image is formed are stacked.

A sheet P received from a sheet receiving section 401 is discharged into an intermediate tray 410 by a conveying roller 407 and pulled back by a bundle return member 409 and the rear end of the sheet P is aligned. After that, alignment members 413a and 413b are moved to the inside to align the side end of the sheet P. The conveying speed of the conveying roller 407 in a color mode is lower than that in a monochrome mode by 20 percent, to avoid jumping from the intermediate tray 410. Further, in the color mode, the alignment members 413a and 413b are reciprocated by three times more than once in the monochrome mode. Thus, the sheet which reacts by the first reciprocation and is out of an alignment position is surely returned to a normal alignment position.