An image forming apparatus main body that allows a first cartridge and a second cartridge to be inserted into an attachment position thereof, characterized by the first cartridge including a first projecting portion and a second projecting portion, the second cartridge including a third projecting portion, which has a shape or a size different from that of the first projecting portion, and a fourth projecting portion, the image forming apparatus main body comprising: a movable member that has a first portion, a second portion and a first recess portion disposed between the first portion and the second portion, and an urging member configured to urge the movable member, wherein, during attachment of the first cartridge to the image forming apparatus main body, the movable member is adapted to be moved from a first position where the first projecting portion is allowed to enter the first recess portion to a second position, by a movement of the first projecting portion in the first recess portion toward a downstream side in an insertion direction of the first cartridge, and allows the second projecting portion to pass through an upstream side of the second portion in a movement direction in which the movable member is moved from the first position to the second position so as to allow the first cartridge to be moved to the attachment position of the image forming apparatus main body, wherein, during attachment of the second cartridge to the image forming apparatus main body, the movable member at the first position is adapted to allow the third projecting portion to enter the first recess portion, and, when the third projecting portion enters the first recess portion to move the movable member from the first position to the second position, the movable member at the second position is adapted to allow the fourth projecting portion to pass through the upstream side of the second portion in the movement direction so as to allow the second cartridge to be moved to the attachment position of the image forming apparatus main body, and wherein the movable member is urged in a direction from the second position to the first position by the urging member. a movable member that has a first portion, a second portion and a first recess portion disposed between the first portion and the second portion, and an urging member configured to urge the movable member, wherein, during attachment of the first cartridge to the image forming apparatus main body, the movable member is adapted to be moved from a first position where the first projecting portion is allowed to enter the first recess portion to a second position, by a movement of the first projecting portion in the first recess portion toward a downstream side in an insertion direction of the first cartridge, and allows the second projecting portion to pass through an upstream side of the second portion in a movement direction in which the movable member is moved from the first position to the second position so as to allow the first cartridge to be moved to the attachment position of the image forming apparatus main body, wherein, during attachment of the second cartridge to the image forming apparatus main body, the movable member at the first position is adapted to allow the third projecting portion to enter the first recess portion, and, when the third projecting portion enters the first recess portion to move the movable member from the first position to the second position, the movable member at the second position is adapted to allow the fourth projecting portion to pass through the upstream side of the second portion in the movement direction so as to allow the second cartridge to be moved to the attachment position of the image forming apparatus main body, and wherein the movable member is urged in a direction from the second position to the first position by the urging member.