A current invention provides a sheet processing apparatus and an image forming apparatus capable of obtaining a sheet bundle which can be double-leaved and released without use of any binding device. A slit S to which a projection Pa provided at an end portion opposite to a side to be bound by a binding portion of a front cover P1 is formed in a back cover P3 of a sheet bundle PA by an insertion portion forming portion. Further, creases L1, L2 for inserting the projection Pa into the slit S while surrounding sheets P2 existing between the front cover P1 and the back cover P2 are formed in the back cover P3 of the sheet bundle PA by a crease forming portion.