A tire air pressure monitoring system has a plurality of sensor units and a receiver. The pressure sensor units are mounted near corresponding tires to measure tire air pressures and transmit transmission signals indicating the respective measured tire air pressures. The receiver has an antenna element, a variable capacitor and a power supply source. The variable capacitor is coupled to the antenna element. The power supply source supplies electric power to the antenna element. The voltage supplied to the variable capacitor is controlled so that a current flowing in the antenna element is changed and its directivity is changed. Thus, the transmission signals will be received more surely.