PURPOSE: To obtain a photosensitive belt module to be easily handled.

CONSTITUTION: When an interchangeable belt module is slid to the direction of a printer 42, a cover having an insertion member is automatically detached from a frame. In the printer 42, the belt is wound around rollers 16, 18 and an encoder roller 44. The belt is wound also around supporting rods 46, 48, 50. The rods 46, 48, 50 are used for accurately positioning developing units for three different colors in the printer 42. When the frame is set up on the printer 42, the operator rotates a lever. The lever rotates around a shaft 36 and expands space between the rollers 16, 18. Thereby the belt is held in a tensed state. When tension is applied to the belt, the printer 42 is turned to a driven state.