A tire air pressure monitoring device for monitoring the air pressure of a tire on a vehicle having two or more tires, including an air pressure sensing device that senses air pressure information of the tire and is adapted to be mounted at a prescribed position on the tire so as to rotate with the tire, a transmitting device that transmits the sensed air pressure information, a rotary position sensing device that senses the rotary position of the air pressure sensing device as the tire rotates, a period of rotation calculating device that calculates the period of rotation of the tire, a period of rotation disruption sensing device that senses disruption in the period of rotation based on a change in the period, and a transmission control device that outputs commands to transmit the air pressure information to the transmitting device based on a disruption in the period of rotation.