An image forming apparatus loadable with a cartridge includes a supporting member, carrying the cartridge, movable between an inside position and an outside position. The supporting member is provided with a first regulating portion to limit the cartridge in one of the longitudinal directions and a second regulating portion to limit movement of the cartridge in the other longitudinal directions. In addition, a third regulating portion limits the supporting member in the one longitudinal direction, and a fourth regulating portion limits the supporting member in the other longitudinal direction. The first, second, third and fourth regulating portions are all provided at the one longitudinal end portion side and outside an image forming region.