To provide a pressure switch free to precisely detect lowering of tire pneumatic pressure in simple constitution and to realize maintenance free eliminating battery exchange, etc. and a tire pneumatic pressure monitoring device using this pressure switch.

The pressure switch 1 to detect tire pneumatic pressure makes electric contact points 4, 5 of the switch contact or non-contact by an air spring 3 to extend and contract in accordance with size of the tire pneumatics. The tire pneumatic pressure monitoring device 20 using the pressure switch 1 arranged in a tire air chamber 34 fixes an electric circuit 6 on a holder 45 rotating synchronously with a tire 33, provides a magnetic pick-up 21 on the car body side in proximity with a rotating locus of the electric circuit 6 and detects contact/non-contact of a movable electrode 4 and a fixed electrode 5 in accordance with output electric power of the magnetic pick-up 21.