A developing unit is pivotally supported by a pivot pin so as to be swingable, and a forcing lever forces a developing roller towards a photosensitive drum so that an outer circumferential surface of a DS roller makes contact with an outer circumferential surface of the photosensitive drum. This maintains, at a specified value, a developing gap between the photosensitive drum and the developing roller. A sympathetic vibration prevention member in which an elastic member is provided on an upper surface of the base member is inserted between a lower portion of a housing of the developing unit and a guide rail provided under the lower portion. This prevents the developing unit from vibrating in sympathetic with vibration during the transportation of the image forming apparatus, which prevents the toner from spilling out.