An image-forming device includes 1st to Mth photosensitive drums (which are arrayed in numeric order in a first direction), a forming unit, an image-carrying member, a detecting unit, and a calibrating unit. Each of the photosensitive drums has a circumferential length D and extends in a first direction. The forming unit forms a registration mark on each of the photosensitive drums. The registration marks are transferred onto the image-carrying member in the numeric order and are arrayed in the first direction. Neighboring registration marks are spaced by a distance L in the first direction. The detecting unit detects positions of the registration marks. The calibrating unit calibrates positions of the photosensitive drums based on the positions detected by the detecting unit. D=NxMxL+(M−1)xL. N being an integer not less than 0.