PURPOSE: To selectively use a belt-like photosensitive body in accordance with user's desire by providing a control means for controlling a paper feeding and image forming process for a seamed belt or the paper feeding and image forming process for a seamless belt based on the detection result of a detection means.

CONSTITUTION: The belt-like photosensitive body 102 and the peripheral device are mounted on a device, and also a process unit 200 capable of freely attaching /detaching with reference to the image forming device main body 100 and a sensor part 201 for detecting whether the belt-like photosensitive body 102 has the seam or not are disposed, and the control of the paper feeding and image forming process for the seamed belt or the control of the paper feeding and image forming process for the seamless belt are performed by a control part 203 based on the detection result of the sensor part 201. Thus, either of the low-priced seamed belt-like photosensitive body or the seamless belt-like photosensitive body with which the image forming process can be performed at high speed can be selectively used in accordance with the user's desire.