A cartridge is detachably insertable into a main assembly of the image forming apparatus. The main assembly includes an opening, and urging, positioning, and regulating portions. The cartridge includes an urgable portion urged by the urging portion in a direction crossing a cartridge-inserting direction after cartridge insertion. The cartridge also includes a positionable portion positioned by contacting the positioning portion by the urging portion moving the cartridge in the crossing direction, and a regulatable portion regulated by the regulating portion to prevent cartridge movement toward the opening when the positionable portion contacts the positioning portion. The regulatable portion is at the cartridge rear and is brought, by cartridge movement in the crossing direction by the urging portion, behind the regulating portion with respect to the inserting direction to prevent cartridge movement toward the opening.