An image forming apparatus for forming an image on a plurality of recording materials (Pa), the image forming apparatus comprising: a main assembly including a rail; a door configured to be movable between a closing position where an opening of the main assembly is closed and an opening position where the opening is open; an accommodating portion for accommodating the plurality of recording materials (Pa); feeding means for feeding the recording materials (Pa) from an end side of the accommodating portion; and a cartridge supporting member provided above the accommodating portion with respect to a vertical direction, wherein the cartridge supporting member is movable, in a direction perpendicular to an axial direction of the feeding means, to a mounting and demounting position (Fig. 6) where a plurality of cartridges is detachably mountable outside the main assembly and to an image forming position (Fig. 2) where the plurality of cartridges is capable of forming the image inside the main assembly, wherein each of the plurality of cartridges includes a photosensitive drum and is detachably mountable to the cartridge supporting member, and wherein at the image forming position (Fig. 2), such a side of the cartridge supporting member as opposes another end side of the accommodating portion with respect to the vertical direction is lower with respect to the vertical direction than such a side of the cartridge supporting member as opposes the end side of the accommodating portion, wherein when the cartridge supporting member is moved from the image forming position (Fig. 2) to the mounting and demounting position (Fig. 6), the rail supports the cartridge supporting member such that the plurality of cartridges held by the cartridge supporting member passes through the opening and the cartridge supporting member is moved obliquely below with respect to the vertical direction, and wherein the rail is configured to be translated in interrelation with the door and the cartridge supporting member is moved upward and downward with raising and lowering of the rail. a main assembly including a rail; a door configured to be movable between a closing position where an opening of the main assembly is closed and an opening position where the opening is open; an accommodating portion for accommodating the plurality of recording materials (Pa); feeding means for feeding the recording materials (Pa) from an end side of the accommodating portion; and a cartridge supporting member provided above the accommodating portion with respect to a vertical direction, wherein the cartridge supporting member is movable, in a direction perpendicular to an axial direction of the feeding means, to a mounting and demounting position (Fig. 6) where a plurality of cartridges is detachably mountable outside the main assembly and to an image forming position (Fig. 2) where the plurality of cartridges is capable of forming the image inside the main assembly, wherein each of the plurality of cartridges includes a photosensitive drum and is detachably mountable to the cartridge supporting member, and wherein at the image forming position (Fig. 2), such a side of the cartridge supporting member as opposes another end side of the accommodating portion with respect to the vertical direction is lower with respect to the vertical direction than such a side of the cartridge supporting member as opposes the end side of the accommodating portion, wherein when the cartridge supporting member is moved from the image forming position (Fig. 2) to the mounting and demounting position (Fig. 6), the rail supports the cartridge supporting member such that the plurality of cartridges held by the cartridge supporting member passes through the opening and the cartridge supporting member is moved obliquely below with respect to the vertical direction, and wherein the rail is configured to be translated in interrelation with the door and the cartridge supporting member is moved upward and downward with raising and lowering of the rail.