A developing device includes a unit support member attached to an opening formed in a front panel of image forming apparatus, support for supporting the unit between the unit support member and a rear panel of the apparatus with the unit pivotable about an axis parallel to the rotation axis of the photoreceptor, and a guide mechanism for guiding a casing of the unit, whereby when the unit is set in the apparatus, a developing sleeve is put in a first state in which the sleeve is near the photoreceptor, and when the unit is inserted in and drawn out of the apparatus, the sleeve is put in a second state in which the sleeve is spaced apart from the photoreceptor. Also when the unit is set in the apparatus, the sleeve is changed from the first state to the second state and vice versa. Accordingly, the unit is replaced through the front panel of the apparatus with ease, and there is eliminated the necessity to provide an excessive space for inserting and drawing the unit in the apparatus, while preventing the developer from unnecessarily coming into contact with the photoreceptor in the non-developing stages, whereby monocolor developing is assuredly performed.