PURPOSE: To facilitate setting switching pressures and its differences by having an auxiliary plate made of magnetic material which, interlocking with a motion of a magnet caused by a change in pressure swing, comes in and out between a lead switch and a magnet, and changes a magnetic state crested by the magnet to the reed switch.

CONSTITUTION: An auxiliary magnetic plate 15, which is located far from a magnet 5 and a reed switch 6 while the pressure is low, is consecutively swung by a support plate 13 and a longitudal plate 14 with a rise of pressure. At this time, when a longitudal plate 14B gets beyond the horizontal line of a frame plate 12A, the auxiliary magnetic plate 15 starts rotating rapidly and its extended part 15A approaches into between the reed switch 6 and a permanent magnet 5. Thus, even if the displacement of the permanent magnet is a little with a subtle change in the pressure, a magnetic state toward the reed switch 6 is converged for effective interruption so that the pressure value for switching can be set properly.