A tire air pressure monitoring system includes a tire air pressure detecting device mounted to each tire of a vehicle having a transmitting section configured to wirelessly transmit necessary data including the tire air pressure data measured by a pressure detecting section at a prescribed interval, and an tire air pressure alarm control device that are mounted in the vehicle and configured to receive the wireless signal transmitted from the tire air pressure detecting device. A transmission control section of the tire air pressure detecting device is configured to initiate a generally continuous transmission at a continuous transmission interval that is shorter than the prescribed interval when the tire air pressure decreases by an amount equal to or greater than a prescribed amount within a prescribed time, and to terminate the generally continuous transmission when the tire air pressure alarm control device is capable of reception of the wireless signal.