A sheet processing apparatus equipped with a saddle stitch stapler is provided with: a driver unit (1) configured to drive a staple; a clincher unit (2) configured to clinch the staple driven by the driver unit (1); a saddle portion (3) integrated with the clincher unit (2); and a guide portion (6) configured to guide the clincher unit (2) toward a predetermined position of the driver unit (1), when a center portion (11) of a bundle of sheets (10) is clamped between the clincher unit (2) and a staple striking portion (21) of the driver unit (1) after the center portion (11) of the bundle of sheets (10) is placed on the saddle portion (3).