In a rotary motion detecting device that outputs a signal corresponding to rotary motion of a rotary disk coupled to a shaft, a boss is fixed to the shaft, the boss is fixed to one surface of the rotary disk, and one of the boss and the shaft is a hole member having a hole formed therein and the other is an inserted member that is inserted in the hole. The hole member has an inner periphery (inner diameter) that is larger than an outer periphery (outer diameter) of the inserted member so that the shaft can be vertical to the rotary disk without being restricted by the boss.