An image forming apparatus for forming an image on a plurality of recording materials (Pa), said image forming apparatus comprising: an accommodating portion for accommodating the plurality of recording materials (Pa); feeding means for feeding the recording materials (Pa) from an end side of said accommodating portion; and a cartridge supporting member provided above said accommodating portion with respect to a vertical direction of said image forming apparatus, wherein said cartridge supporting member is movable, in a direction perpendicular to an axial direction of said feeding means, to a mounting and demounting position (Fig. 6) where a plurality of cartridges is detachably mountable outside a main assembly of said image forming apparatus 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 cartridges is detachably mountable to said cartridge supporting member, and either each cartridge includes a photosensitive drum or said cartridge supporting member includes photosensitive drums, and a transfer member (12, 13) onto which a developer image formed on the photosensitive drums is to be transferred, wherein when said cartridge supporting member is moved from the image forming position (Fig. 2) to the mounting and demounting position (Fig. 6), said cartridge supporting member is moved obliquely downward with respect to the vertical direction, characterized in that at the image forming position (Fig. 2), such a side of said cartridge supporting member as opposes another end side of said accommodating portion with respect to the vertical direction is lower with respect to the vertical direction than such a side of said cartridge supporting member as opposes the end side of said accommodating portion, and a surface of said transfer member (12, 13) which comes into contact with the photosensitive drums is provided in an obliquely inclined state at the image forming position (Fig. 2), and is provided between said cartridge supporting member and said accommodating portion with respect to the vertical direction. an accommodating portion for accommodating the plurality of recording materials (Pa); feeding means for feeding the recording materials (Pa) from an end side of said accommodating portion; and a cartridge supporting member provided above said accommodating portion with respect to a vertical direction of said image forming apparatus, wherein said cartridge supporting member is movable, in a direction perpendicular to an axial direction of said feeding means, to a mounting and demounting position (Fig. 6) where a plurality of cartridges is detachably mountable outside a main assembly of said image forming apparatus 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 cartridges is detachably mountable to said cartridge supporting member, and either each cartridge includes a photosensitive drum or said cartridge supporting member includes photosensitive drums, and a transfer member (12, 13) onto which a developer image formed on the photosensitive drums is to be transferred, wherein when said cartridge supporting member is moved from the image forming position (Fig. 2) to the mounting and demounting position (Fig. 6), said cartridge supporting member is moved obliquely downward with respect to the vertical direction, characterized in that at the image forming position (Fig. 2), such a side of said cartridge supporting member as opposes another end side of said accommodating portion with respect to the vertical direction is lower with respect to the vertical direction than such a side of said cartridge supporting member as opposes the end side of said accommodating portion, and a surface of said transfer member (12, 13) which comes into contact with the photosensitive drums is provided in an obliquely inclined state at the image forming position (Fig. 2), and is provided between said cartridge supporting member and said accommodating portion with respect to the vertical direction.