A regulating plate (26) for regulating the amount of magnetic flux in a magnetic path through which magnetic flux generated from a permanent magnet (23) is allowed to pass is arranged such that the plate is abuttable on a back plate (15) in parallel to a movable shaft (1). A part (26a) of the regulating plate (26) adjacent to the back plate (15) is shaped such that the width, or surface area of the part (26a) is smaller than that of a part (26b) of the regulating plate adjacent to an isolation spring (8). Magnetic saturation is caused in the part (26b) of the regulating plate so that the amount of magnetic flux of the permanent magnet (23) is less than a releasing force from the isolation spring (8), thus regulating the amount of magnetic flux passing.