A saddle stitching binding device includes an additional folding roller unit that presses a fold line portion of a folded sheet bundle and performs additional folding, and a unit movement mechanism that causes the additional folding roller unit to reciprocate in the width direction of the sheet bundle. The saddle stitching binding device further includes a sheet supporting device that is located on a movement path of the additional folding roller unit and supports the lower surface side of the sheet bundle when the sheet bundle after pressed is conveyed to the downstream side. In the saddle stitching binding device, a supporting position at which the sheet supporting device supports the sheet bundle and a retreat position at which the sheet supporting device permits the additional folding roller unit to move are set to the sheet supporting device.