Provided is a sheet processing apparatus connected to an image forming apparatus, including: an intermediate process tray which receives sheets delivered from the image forming apparatus as a sheet stack; a process unit which applies a predetermined process for the sheet stack received on the intermediate process tray; a pair of rollers which nips and delivers the sheet stack subjected to the predetermined process by the process unit; a stack tray which receives the sheet stack delivered by the pair of rollers; and a controller moves one roller of the pair of rollers to a standby position corresponding to a thickness of the sheet stack to be delivered onto the stack tray.