Belt unit and image forming apparatus employing same

A belt unit is detachably attached to a housing. A holder changes a state of an endless belt by moving a transfer roller between a first state as an endless belt movement state and a second state as a detachment state. The first state allows the endless belt to circulate for image formation, and the second state allows the belt unit to be detached from the housing. The holder moves the transfer roller adjacent supporter independently of the transfer roller and lifts the opening in the endless belt to a prescribed position to expose it to the front side plate.