To enable reliable oblique stapling without involving increase in a motor current, to enable oblique stapling at a wide range from a center to end portions and to enable oblique stapling for various sheet sizes.

A sheet processing apparatus includes: a stapler 8; a slider 16 on which the stapler 8 is mounted through a sector gear 19; a motor with a pulley 23 and a timing belt 21 linearly moving the slider 16 along a guide shaft 15; a gear 17 provided in the slider 16 and including an abutting member 18 which is meshed with the sector gear 19 to change the posture of the stapler 8; a hole 40 for detachably holding the posture of the stapler 8 in a plurality of different positions; a holding member 41 elastically fitted into the hole 40; an abutted member 24 against which the abutting member 18 is abutted and which changes the posture of the held stapler 8; and plate-like restricting members 25a and 5b arranged in parallel with the movement direction of the stapler 8 and restricting the position of the abutting member 18 so as to maintain the held posture.