To provide a booklet forming apparatus that enables obtaining a saddle-stitched booklet in which positions of the needle agree to the centerline in the thickness direction of the back of the booklet after the forming.

In the booklet forming apparatus that has a booklet forming means that holds the saddle-stitched booklet that has been sent, presses the back side, and forms a rectangle, either the front cover side or the back cover side of the saddle-stitched booklet is sent off to the carrying direction before the saddle-stitched booklet is held, and, meanwhile the posture of the booklet when held is changed with the carrying direction by pulling it back to the reverse. When the needle position shifts from the center of the back after the forming, regulation is carried out by this posture change and the binding needle may come to the center of the back after the forming.