A sheet processing device includes a stapling tray, a rear-end clamp, and a stapler that staples sheets while the rear-end clamp is clamping the sheets. In response to a start signal, the stapler moves to a position where the stapler does not interfere the rear-end clamp while being closest to a stapling position. When the last sheet of a sheet bundle to be stapled is conveyed into the stapling tray, timing at which the stapler moves to the stapling position is determined according to a preset condition.