A sheet post-processing device of the present disclosure is provided with a punch hole forming device, a processing tray, an ejection tray, a conveyance roller pair, an ejection roller pair, and a stiffening member. In a case where a sheet through which punch holes have been formed is to be ejected on the ejection tray without being subjected to post-processing on the processing tray, in a state where the stiffening member is retracted in a direction away from the sheet, the conveyance roller pair conveys the sheet until at least a tip end thereof reaches on the ejection tray.