A transmitter device includes a rotational direction sensor that generates a specific pattern signal when a corresponding tire rotates. The pattern signal of the left tires is different from the pattern signal of the right tires. A front reception antenna is near the front tires. A rear reception antenna is near the rear tires. A receiver includes a switch circuit that selectively connects the front reception antenna or the rear reception antenna to a controller. When one transmitter device transmits a radio signal, the controller determines which transmitter device is the source of the radio signal in accordance with the level of the signals from the reception antennas and the pattern signal, which is included in the data transmitted by the transmitter device.