PURPOSE: To reduce the size and weight of a motor-driven stop device and to control the stop tool closely by forming a 1st gear plate of a magnetic body and eliminating the need to arrange a magnet on a surface facing a 2nd gear plate, and driving the stop tool through the differential rotation between the 1st and 2nd gear plates.

CONSTITUTION: When a control current flows through winding 16 in order, magnetic flux is produced in the axial direction of the 2nd gear plate 15 to attract the 1st gear plate 11 and 2nd gear plate 15, and a 1st tooth part 11b and a 2nd tooth part 15a engage each other in order. Then the 1st and 2nd gear plates 11 and 15 differ in the number of teeth and one is restrained from rotating, so the other gear plate which is restrained rotates to drive the stop tool 18, thereby opening and closing an optical path through the sliding of stop blades. Further, the 1st gear plate 11 is made of the discoid annular magnetic body and a magnet need not be arranged separately, so the whole device never increases in size axially.