A transfer belt unit which is mounted in an image forming device. The transfer belt unit contains a positioning member; a handle, and a guiding member. The positioning member positions the transfer belt unit with respect to the image forming device in a state where the transfer belt unit is inserted into a guide unit. The handle is placed on the rear side of the positioning member in a direction of inserting the transfer belt unit into the guide unit. The guiding member holds the transfer belt unit when the transfer belt unit is inserted into the guide unit by abutting on a guide surface of the guide unit together with the positioning member before the handle is inserted into the guide unit.