A bookbinding apparatus, having a sheet bundle storing device (40) to store a sheet bundle, a grasping device to grasp the sheet bundle stored in the sheet bundle storing device (40), a square spine forming device to form a square spine, wherein a pair of forming members bend a coversheet bonded onto a back section of the sheet bundle grasped by the grasping device, a pressure force controlling device to vary a pressure force of the pair of forming members to press a bending section of the coversheet in accordance with bookbinding conditions.