To monitor tire pressure constantly and keep the service life of a battery long by providing a switch disposed between a detector and the battery for supplying electric power to the detector, and also providing a timer for switching the switch on and off.

A switch 32 is provided between a battery 31 and a detector 20, and a timer 33 switches the switch 32 on and off at constant intervals. The timer 33 switches the switch 32 on for 0.1 second at intervals of 120 seconds, for instance. Electric power is thereby supplied to the detector 20 from the battery 31. A control part 24 takes in pressure data from a pressure sensor 21 and compares this pressure data with the preceding pressure data stored in a memory 23. In the case of the compared result being different, a processed signal is outputted from an antenna 27. After the processed signal is received and processed in a receiver 40, tire pressure is displayed on a display part 47. That is, the switch 32 is switched on and off by the timer 33, and electric power is supplied to the detector 20 from the battery 31 to constantly detect tire pressure.