PURPOSE: To monitor corresponding to the speed at the minimum pressure by providing a mass part to be controlled by the centrifugal force in a conductive switching diaphragm for forming a reference pressure chamber, and automatically varying a switching point of a pressure switch.

CONSTITUTION: This pressure switch 1 to be arranged in a wheel rim 2 has a pressure chamber 6 provided with the reference pressure in a sensor casing 5, and this reference pressure is sealed in relation to the tire pressure by a switching diaphragm 7, and a contact pin 8 is provided at a central part of the pressure chamber 6. An additional mass is fitted to the diaphragm 7, or a thick (d) switching diaphragm 7a is formed so as to automatically trace the switching point of the pressure switch 1 in response to the speed. With this structure, switching point of the pressure switch 1 is varied so as to adapt to the pressure necessary to tires in response to each speed to realize monitor corresponding to the speed.