Disclosed is a sheet post-processing device, including: a downstream side intermediate stacking unit having a pair of slide guides for supporting left and right edges of the sheet that is sent, the slide guides moving toward and away from each other so that the sheet is pressed against one of the slide guides that serves as a reference by the other slide guide to be positioned; a processing unit for processing the sheet positioned by the downstream side intermediate stacking unit; and a stacking unit for receiving the sheet that is dropped through a gap created between the slide guides, in which the pair of slide guides each have a bottom piece for supporting the edge of the sheet, and in which the bottom piece of the other slide guide is partially cut off to provide a remaining portion, the remaining portion making the gap between the bottom pieces wider on the upstream side than on the downstream side in the sheet conveying direction.