A tandem type color printer, having optical beam scanning units and corresponding photosensitive drums, which detects a positional shift of a scanning locus occurring on a corresponding one of the plurality of photosensitive drums, and automatically corrects the detected positional shift of the scanning locus. A detecting window, in which a beam transmission time varies with a change in the beam scanning position, is provided at each of the start end side and the terminal end side of each of a plurality of beam scanning loci in a mask plate. The mask plate is arranged between the optical beam scanning units and the photosensitive drums, to provide output of a detection signal corresponding to the incident time of the transmission beam resulting from beam scanning by a light source, and thus determines and corrects the direction and the amount of positional shift of the scanning locus.