In the developing apparatus including a developing bath and a toner transport pipe for supplying toner to the developing bath, a toner supply portion opening/closing mechanism is provided, that includes a shutter member for opening and closing a toner supply port of the toner transport pipe that is contained in a base portion of the toner transport pipe, a shaft for moving the shutter member back and forth in a longitudinal direction of the developing bath, a second spring member provided such that the shaft is inserted therein, a first spring member for supporting the shaft detachably to the shutter member through its extension and contraction, and a lever member for extending and contracting the first spring member through its releasing and closing.