To provide an image forming device which has an image reader on its top and can improve the maintenance by making it easy to load a cartridge having a development unit and a belt holder having a belt.

This image forming device is built in a way that it can load a cartridge 30 having a development unit 31 and a belt holder 59 which is in contact with the cartridge and retaining a belt 58 almost horizontally by using a guide mechanism 116. The guide mechanism has a clamp which retains the cartridge or the belt holder with a spring 103 and releases them when removing. Since the cartridge or the belt holder is pulled out horizontally, they can be replaced without interfering with the image reader on top of this image forming device. In this configuration, the cartridge or the belt holder can be loaded easily improving the maintenance of this device.