A developing device detachably mountable to a main assembly of the electrophotographic image forming apparatus, said developing device comprising: a developing roller for developing an electrostatic latent image formed on an electrophotographic photosensitive drum with a developer; a shaft coupling member, for transmitting a rotational driving force from a rotatable main assembly driving force transmission member provided on said main assembly of the apparatus with a deviation permitted between an axis of the main assembly driving force transmission member and an axis of said developing roller, wherein said shaft coupling member includes an engaging portion for engaging with the main assembly driving force transmission member and receiving the rotational driving force, when said developing device is mounted to the main assembly of the apparatus; said engaging portion is movable in a direction crossing with the axial direction of said developing roller; characterized in that said shaft coupling member is provided on one axial end of said developing roller, and the developing device is adapted such that when said developing device enters said main assembly of the apparatus, said engaging portion is positioned to a holding portion of and provided on said developing device; when said developing device is positioned in said main assembly, by movement of a movable member of said main assembly from a first position to a second position, said engaging portion is positioned to a main assembly locking member provided on said main assembly of the apparatus; and a distance (D1, D2, D3, D4, D5; S1, S2, S3, S4; L1, L2, L3) between an axis (48c5) of said engaging portion which is substantially parallel to an axis of said developing roller, and the axis of said developing roller is reduced when said engaging portion is positioned by said main assembly locking member than when said engaging portion is positioned by said holding portion, wherein said developing device is provided with an urging member for urging said engaging portion in the direction crossing with its axis (48c5) or with an elastically deformable elastic portion such that the engaging portion is positioned to said holding portion. a developing roller for developing an electrostatic latent image formed on an electrophotographic photosensitive drum with a developer; a shaft coupling member, for transmitting a rotational driving force from a rotatable main assembly driving force transmission member provided on said main assembly of the apparatus with a deviation permitted between an axis of the main assembly driving force transmission member and an axis of said developing roller, wherein said shaft coupling member includes an engaging portion for engaging with the main assembly driving force transmission member and receiving the rotational driving force, when said developing device is mounted to the main assembly of the apparatus; said engaging portion is movable in a direction crossing with the axial direction of said developing roller; characterized in that said shaft coupling member is provided on one axial end of said developing roller, and the developing device is adapted such that when said developing device enters said main assembly of the apparatus, said engaging portion is positioned to a holding portion of and provided on said developing device; when said developing device is positioned in said main assembly, by movement of a movable member of said main assembly from a first position to a second position, said engaging portion is positioned to a main assembly locking member provided on said main assembly of the apparatus; and a distance (D1, D2, D3, D4, D5; S1, S2, S3, S4; L1, L2, L3) between an axis (48c5) of said engaging portion which is substantially parallel to an axis of said developing roller, and the axis of said developing roller is reduced when said engaging portion is positioned by said main assembly locking member than when said engaging portion is positioned by said holding portion, wherein said developing device is provided with an urging member for urging said engaging portion in the direction crossing with its axis (48c5) or with an elastically deformable elastic portion such that the engaging portion is positioned to said holding portion.