Each of a plurality of tire air pressure detectors includes an information retention portion that retains one or more pieces of timing information indicating the time at which the tire air pressure detector has rotated to a specific detector angle in a first time band for withholding transmission of a radio wave signal, and a radio wave transmission control portion that transmits one or more of the pieces of timing information retained in the information retention portion to a receiver in a second time band enabling transmission of the radio wave signal. The receiver includes an axle rotation amount reader portion that receives axle rotation amount information generated in each of a plurality of axle rotation detection portions, and a position determination portion that calculates an axle rotation amount at the point in time at which the tire air pressure detectors have rotated to the specific detector angle.