To dispense with initial registering work by automatically specifying a transmitter as a transmission source.

The transmitters have a rotational direction sensor for generating a specific pattern signal following the rotation of a corresponding tire. A pattern signal generated by a rotational direction sensor of the transmitter in a left tire is different from a pattern signal generated by a rotational direction sensor of the transmitter in a right tire. A front-side receiving antenna 21 corresponds to right and left front tires. A back-side receiving antenna 22 corresponds to right and left back tires. A receiver 5 has a changeover circuit 23 for alternatively connecting both receiving antennas 21 and 22 to a controller 20. When one of the transmitters transmits radio wave, the controller 20 specifies the transmitter as the transmission source based on levels of respective signals inputted from both receiving antennas 21 and 22 and data indicating a pattern signal included in the transmitted data from the transmitter.