It is determined whether light has passed through apertures in a seconds, a center and an intermediate wheels and a relevant one of apertures in an hour wheel, thereby determining their respective rotational positions. When the center wheel rotates one step at a time in a predetermined direction to such a position that the apertures in the center and intermediate wheels align and a detection unit detects light which has passed thorough the apertures, the center wheel is returned twelve steps or more in the reverse direction. The center wheel is again rotated one step at a time in the predetermined direction to the position where the detection unit detected light first. When the detection unit again detects light at that position, this position is determined as the reference position in the center wheel.