PURPOSE: To obtain a dichroic copy of high quality without color deviation while eliminating variation in path length between transfer positions of both photosensitive drums by holding the distance a transfer belt and a separation pawl constant even when the transfer belt has variance in peripheral length.

CONSTITUTION: Pins 24 and 25 are implanted in a frame 17 and engaged with long-sized holes of a frame 18 to provide the frame in parallel to the common tangent connecting transfer parts of both photosensitive drums, and the frame 17 is allowed to move to and away from the frame 18 in parallel to the direction in which there should be the top surface of the belt. Further, a recessed part which allows a bearing 19 which support a shaft end of a driven roller 7 to slide only; in the common tangent direction is formed at the left end of the frame 17, and the bearing 19 is energized by a compression spring 15 in such a direction that tension is applied to the belt. Therefore, the tensile strength of the belt balances with the compressive force of the spring 15 and the bearing 19 shifts to right or left according to the difference in belt peripheral length, but the outer circumferential uppermost part of the driven roller is invariably on the common tangent.