When a revolution speed of the engine changes faster or slower, the vibration from the engine is propagated to an occupant in the vehicle, and the occupant may feel a sense of discomfort. At least the permanent magnet is surrounded by ferromagnetic material, and the spacing of the permanent magnet from the ferromagnetic material changes upon imposition of a force upon the pickup element. The crotch belt runs through the positioner and is connected to the seat shell with the belt attachment. This transforms the lower trench portions to expanded, spheroidal voids that extend under the device sites.