In an image-forming device, a belt unit is detachably mounted in the housing. The belt unit moves to an installation position along a linear insertion/removal path in an insertion direction when the belt unit is mounted in the housing. The belt unit includes: a plurality of rollers; and an endless belt that is supported by the plurality of rollers and is capable of moving circumferentially around the plurality of rollers. A surface of the endless belt contacts a surface of the at least one photosensitive member when the belt unit is located in the installation position. The endless belt moves in a belt-moving direction that is angularly shifted from the removal direction to allow the surface of the endless belt to separate from each photosensitive member when the belt unit starts moving in the removal direction from the installation position.