PURPOSE: To achieve accurate and secure detection at rotational frequencies over a wide range by making the area of at lease one detected part on one place side than that of an adjacent one among detected parts for angular displacement detection formed from one prescribed place on a rotary disk at intervals.

CONSTITUTION: Many detected parts 15 for angular displacement detection are formed, startaing at one predetermined place P, on a rotary disk 13 at prescribed intervals in the opposite rotational directions of both driving and driven shafts 1 and 8 over a wide range greater than angular displacement when a maximum load is applied to a rotary motion transfer means 12 between the driving and driven shafts 1 and 8. Then, the area of one detected part 15a on the side of one place P is made greater than that of other adjacent detected parts 15, and even when the rotary disk 13 rotates fast, the passing of a starting detected part 15a having greater area increases the quantity of photodetection by a photocoupler 22, starting counting operation securely.