A developing device detachably mountable to a main assembly of an image forming apparatus includes a developing roller for developing an electrostatic latent image formed on an electrophotographic photosensitive member, with the developing roller including an elastic roller portion and a core metal portion for supporting the roller portion, a developing device frame for rotatably supporting the developing roller, and a developer layer thickness regulating member, contacting the roller portion of the developing roller, for regulating a layer thickness of a developer carried on the developing roller. In addition, a movement preventing portion prevents movement of the developing roller in an axial direction, wherein the movement preventing portion is detachably mounted in the developing device and is inserted into a spacing between an end portion of the core metal portion and an opposing portion of the developing device frame opposing the end portion of the core metal portion with respect to the axial direction of the developing roller. The movement preventing portion is removed from the developing device when the developing roller develops the electrostatic latent image.