An image forming apparatus may include a removable/attachable belt unit. In operation, the belt unit may be secured to a main body of the image forming apparatus using a lock mechanism to prevent extraneous or unexpected shifts and/or other movements. The lock mechanism may be located closer to a rotation pivot than to a midpoint between the rotation pivot and a rotation axis of a driven roller. This positioning of the lock mechanism may allow easier attachment and removal of the belt unit. Additionally, the lock mechanism may include an urging member configured to sandwich a protruding member of the belt unit against a contact surface in a front-rear direction. This reduces the potential for elastic or creep deformation of the belt unit.