An image-forming device has cylindrical image-carrying members, and an optical scanning unit. The cylindrical image-carrying members are in a main casing for rotating about respective rotational axes which are parallel to one another and juxtaposed in a single direction. The optical scanning unit is in the main casing and has laser generators, a rotatable polygon mirror, mirrors, and lenses. The laser generators have a one-to-one relationship to the image-carrying members to emit laser beams. The rotatable polygon mirror deflects the respective laser beams to scan the respective image-carrying members. The mirrors have a one-to-one relationship to the image-carrying members to guide the respective laser beams along respective optical paths to the respective image-carrying members. The lenses have a one-to-one relationship to the plurality of image-carrying members provided in the respective optical paths and between the respective mirrors and the respective image-carrying members.