PURPOSE: To simplify detaching and attaching operations of a developing device by sliding the developing device to a rotary drum side by a spring, in a state that a detaching and attaching member turned to a reference position and an engaging pin are engaged.

CONSTITUTION: A surface to be made to contact by pressing for controlling a horizontal position of a developing device 43, and grooves 505, 506 being a guide part for regulating the upper and lower positions are provided on a supporting frame body 5 for supporting a rotary drum 41 so as to be freely rotatable. Also, apart from this supporting body 5, a detaching and attaching member 509 which can turn in one direction from a reference position and also can move to the rotary drum 41 side in the reference position is provided by energizing it to the rotary drum 41 side at all times. Also, an engaging part for moving the developing device 43 as one body toward the rotary drum 41 is provided by being engaged to an engaging part provided on a prescribed position of the detaching and attaching member 509.