Patent ID: 8346139

Claim:
A belt unit comprising: a fixed frame at which a transfer roller is provided; a plurality of fixed side supporting rollers provided at the fixed frame; a movable frame that is swingably mounted to the fixed frame, and at which a plurality of transfer rollers are provided; a movable side supporting roller provided at the movable frame; and an endless belt stretched around the fixed side supporting rollers and the movable side supporting roller, and contacting the transfer roller provided at the fixed frame and the transfer rollers supported at the movable frame, and contacting a plurality of photoreceptor drums, wherein a swinging center of the movable frame is a center of belt portions at both sides of the swinging center, and in a monochrome mode, the movable frame is swung and moves the belt away from the photoreceptor drums other than the photoreceptor drum that is used in the monochrome mode, wherein two rollers are provided as the fixed side supporting rollers, and the swinging center is positioned on a line that connects a central point of a line connecting rotational centers of the two rollers, and a rotational center of the movable side supporting roller in a color mode.