A sheet post-processing device is connected to a punch processing unit. The punch processing unit includes a skew detection unit and a punch unit. The skew detection unit detects a skew amount of a sheet. The punch unit forms punch holes at positions of the sheet according to the skew amount. The sheet post-processing device includes a processing tray, a paddle, a driver, and a control unit. The sheet of which the skew amount is detected by the skew detection unit is mounted on the processing tray. The paddle conveys the sheet on the processing tray by a paddle operation. The driver drives the paddle to perform the paddle operation. The control unit is configured to operate the driver to control a number of times the paddle operation is performed by the paddle based on the skew amount.