PURPOSE: To reliably apply braking torque to an output shaft when a housing suddenly rotates with the output shaft as the center by forming a spring element to surround the output shaft out of a helical spring.

CONSTITUTION: Inertial mass 14 coaxial with an output shaft 5 is rotatably arranged in a housing 1 through a ball bearing 15. This inertial mass 14 is joined to the housing 1 through a helical spring 16 to surround the output shaft 5. This helical spring 16 exerts braking torque on the output shaft 5 by contacting with the output shaft 5 by diametrically contracting when the housing 1 relatively rotates in one rotational direction to the inertial mass 14. As a result, an overload joint formed of a gear 10 and a disc spring 11 is released by the braking torque.