Patent ID: 7260497

Claim:
A tire air pressure monitoring system, comprising: a plurality of tire air pressure sending units which are respectively attached to wheels of a trailer of a vehicle to send a radio wave containing tire air pressure information of the wheels by radio; a plurality of antennas for receiving the radio wave; and a tire air pressure receiving apparatus for acquiring the tire air pressure information from a signal which is based on the radio wave received by the antennas, the tire air pressure receiving apparatus being mounted at a tractor head of the vehicle, wherein: each of the antennas is a directional antenna, which is mounted at the tractor head and oriented to have a receiving sensitivity in a direction toward the trailer which is stronger than that in other directions; and the plurality of antennas are respectively mounted at different positions in a left-right direction of the vehicle so that an overlap portion where receiving fields of the antennas overlap each other has a spatial diversity effect, and the plurality of antennas are arranged in such a manner that the receiving fields thereof cover all of the wheels of the trailer.