Tire air pressure monitoring device

A tire air pressure monitoring device includes a sensor (5) which detects a physical quantity relating to tire air pressure, and a transmitter (8) which sends signals detected by the sensor (5). The sensor (5) is a sound pressure level sensor which detects a sound pressure level inside a tire cavity filled with air. The transmitter (8) is enabled to send the signals only when the sound pressure level detected by the sound pressure level sensor is changed.