An image forming apparatus for forming an image on a recording material, comprising: a supporting member for supporting a plurality of cartridges, wherein said supporting member is movable to an outside position () to which said supporting member is capable of being pulled out of a main assembly of said image forming apparatus and where the cartridges are detachably mountable to said supporting member, a first inside position () where the cartridges are positioned to the main assembly in an inside of the main assembly, and a second inside position () where said supporting member is movable to the outside position () and the first inside position () in the inside of the main assembly; a first positioning portion (51a, 52a), provided in the main assembly, for positioning an associated cartridge of the cartridges when said supporting member is located at the first inside position (), and a first preventing portion, provided in the main assembly, for preventing rotation of the associated cartridge about said first positioning portion (51a, 52a); and a second positioning portion (70a, 70b), provided to said supporting member, for positioning the associated cartridge when said supporting member is located between the outside position () and the second inside position (), and a second preventing portion, provided to said supporting member, for preventing rotation of the associated cartridge about said second positioning portion (70a, 70b), characterized in that, in a state in which said supporting member is located at the first inside position said second preventing portion and said second positioning portion (70a, 70b) are spaced from said associated cartridge, by which the support of each cartridge by the supporting member is eliminated. a supporting member for supporting a plurality of cartridges, wherein said supporting member is movable to an outside position () to which said supporting member is capable of being pulled out of a main assembly of said image forming apparatus and where the cartridges are detachably mountable to said supporting member, a first inside position () where the cartridges are positioned to the main assembly in an inside of the main assembly, and a second inside position () where said supporting member is movable to the outside position () and the first inside position () in the inside of the main assembly; a first positioning portion (51a, 52a), provided in the main assembly, for positioning an associated cartridge of the cartridges when said supporting member is located at the first inside position (), and a first preventing portion, provided in the main assembly, for preventing rotation of the associated cartridge about said first positioning portion (51a, 52a); and a second positioning portion (70a, 70b), provided to said supporting member, for positioning the associated cartridge when said supporting member is located between the outside position () and the second inside position (), and a second preventing portion, provided to said supporting member, for preventing rotation of the associated cartridge about said second positioning portion (70a, 70b), characterized in that, in a state in which said supporting member is located at the first inside position said second preventing portion and said second positioning portion (70a, 70b) are spaced from said associated cartridge, by which the support of each cartridge by the supporting member is eliminated.