A developing device usable with an electrophotographic image forming apparatus, said apparatus including a driving shaft rotatable having a rotating force applying portion and not movable in a direction substantially perpendicular to an axial direction of said driving shaft, and a rotatable rotary, said developing device being movable in a direction substantially perpendicular to an axial direction of the driving shaft in response to rotation of said rotary with said developing device mounted to said rotary, said developing device comprising: i) a frame (113, 138); ii) a developing roller, for developing an electrostatic latent image formed on an electrophotographic photosensitive member drum, rotatably supported by said frame (113, 138); iii) a developer accommodating portion for accommodating a developer for use by said developing roller to developing the electrostatic latent image; iv) a coupling member for transmitting a rotating force to said developing roller in a state that developing cartridge is mounted to said rotary, said coupling member including (i) a rotating force receiving portion for engaging with the rotating force applying portion to receive the rotating force from the driving shaft and (ii) a projected portion at a trailing end which is opposite from said rotating force receiving portion; v) a regulating member provided on said frame (113, 138), the regulating member including (i) a regulating portion (160b1) for regulating said coupling member at a pre-engagement angular position before engagement with the driving shaft by contacting with said projection, (ii) a relatively narrow portion (160b7)and (iii) a permitting portion (160b2), for permitting said coupling member to revolve substantially, provided with a relatively wide portion (160b8); vi) an elastic member for elastically urging said coupling member to position said coupling member at the regulating portion, wherein said developing device is adapted such that, when said rotary rotates, said coupling member moves from the pre-engagement angular position to a rotational force transmitting angular position in response to movement of said coupling member from the regulating portion to the permitting portion against an elastic force of said elastic member by said coupling member contacting said driving shaft member with movement of said developing device, and when said rotary further rotates, said coupling member moves from the rotational force transmitting angular position to a disengaging angular position to disengage said coupling member from the driving shaft against an elastic force of said elastic member, and wherein when said projected portion is in said narrow portion, an inclining direction of said coupling member is regulated at the pre-engagement angular position, and when said projected portion in the wide portion, said coupling member is substantially revolvable. i) a frame (113, 138); ii) a developing roller, for developing an electrostatic latent image formed on an electrophotographic photosensitive member drum, rotatably supported by said frame (113, 138); iii) a developer accommodating portion for accommodating a developer for use by said developing roller to developing the electrostatic latent image; iv) a coupling member for transmitting a rotating force to said developing roller in a state that developing cartridge is mounted to said rotary, said coupling member including (i) a rotating force receiving portion for engaging with the rotating force applying portion to receive the rotating force from the driving shaft and (ii) a projected portion at a trailing end which is opposite from said rotating force receiving portion; v) a regulating member provided on said frame (113, 138), the regulating member including (i) a regulating portion (160b1) for regulating said coupling member at a pre-engagement angular position before engagement with the driving shaft by contacting with said projection, (ii) a relatively narrow portion (160b7)and (iii) a permitting portion (160b2), for permitting said coupling member to revolve substantially, provided with a relatively wide portion (160b8); vi) an elastic member for elastically urging said coupling member to position said coupling member at the regulating portion, wherein said developing device is adapted such that, when said rotary rotates, said coupling member moves from the pre-engagement angular position to a rotational force transmitting angular position in response to movement of said coupling member from the regulating portion to the permitting portion against an elastic force of said elastic member by said coupling member contacting said driving shaft member with movement of said developing device, and when said rotary further rotates, said coupling member moves from the rotational force transmitting angular position to a disengaging angular position to disengage said coupling member from the driving shaft against an elastic force of said elastic member, and wherein when said projected portion is in said narrow portion, an inclining direction of said coupling member is regulated at the pre-engagement angular position, and when said projected portion in the wide portion, said coupling member is substantially revolvable.