Pneumatic pressure monitoring units that measure pneumatic pressures of respective tires and that transmit results of the measurements of the pneumatic pressures from respective antennas by the use of radio signals are provided at the tires, respectively. A communication unit including a beam controller arranged to selectively direct reception beams towards the respective tires is provided at substantially the center near the bottom surface of a vehicle. The antennas of the respective pneumatic pressure monitoring units are constructed so that directivity patterns of transmission beams are directed towards the inside in axle directions of the tires.