An image forming apparatus comprising: a built-in rotary driving member; a developing device detachably attachable to the body of the image forming apparatus; a developer driving part disposed at one end in a shaft direction of the developing roller and the agitation screw; a driving part duct enveloping the developer driving part of the developing device; a developer case surface duct formed along a surface parallel to the shaft direction of the developing roller and the agitation screw; and a blast fan or a exhaust fan to blow air from either the driving part duct or the developer case surface duct to each other.