PURPOSE: To surely detect a shock by a method wherein, when a ring-shaped magnet is subjected to the shock, a reed switch is made to perform a sensing operation and the ring-shaped magnet is held within a definite feedback sensing sphere by a frictional coupling force acting on a taper.

CONSTITUTION: The diameter of the whole circumference of a sealing tube 23 which seals a reed switch 24 is expanded gradually in the axial direction to form a taper outer-circumferential wall 23a the diameter of the whole circumference of also a play fitting hole 26a in a ring-shaped magnet 26 is reduced in its diameter gradualy in the axial direction, and a taper inner-circumferential wall is worked. A shock sensor 21 acts in such a way that the movement in the axial direction of the ring- shaped magnet 26 which is play-fitted to the sealing tube 23 as the outer shell of the reed switch 24 is regulated by the coupling operation of the taper outer- circumferential wall 23a to the taper innercircumferential wall of the play fitting hole 26a within a sensing sphere, the taper parts are coupled frictionally to each other on the whole circumference nearly over the whole length in the axial direction. Thereby, the ring-shaped magnet 26 can be stopped within the sensing sphere by a frictional coupling force corresponding to the inclination of the taper parts so as to make a soft landing.