<p>Air pressure monitoring units (11A-11D) are individually attached to tires (10A-10D) and measure the air pressure of the tires to transmit the result by radio from antennas. A communication unit (12) is installed at substantially the center near the bottom surface of a vehicle (9). The communication unit (12) has a beam control means for selectively directing reception beams (BRA-BRD) in directions of the individual tires. The antennas of the air pressure monitoring units are constructed so that directive patterns (BTA-BTD) of transmission beams are directed toward the inside in the directions of axles of the tires.</p>
