PURPOSE: To prevent toner from being leaked from a developing device unit and to prevent soiling by toner from occurring by moving many developing device units so that one of them is positioned within a developing band, separating the other developing device units from the developing band and hermetically sealing them.

CONSTITUTION: The developing device unit 24 is set at an actuating position and the developing device units 18, 20 and 22 are set at a non-actuating position. By a shield, that means, a cover 54 arranged on one side of the developing band, the non-actuating units 18, 20 and 22 arranged on the other side of the developing band are hermetically sealed. By a shield, that means, a cover 56 arranged on the other side of the developing band, the developing device unit moved to the other side is hermetically sealed. By a motor 48 coupled to a developing device carriage 50, the units 18, 20, 22 and 24 are horizontally moved in parallel between the non-actuating position and the actuating position like it is shown by an arrow 26.