Provided is a tire air pressure monitoring device capable of accurately determining the wheel position of a transmitter. The tire air pressure monitoring device comprises: a transmitter installed on each wheel for transmitting detected air pressure information via a wireless signal; a rotational position detection mechanism disposed on the vehicle body side which detects the rotational position of each wheel and also outputs rotational position information to a communications line at prescribed time intervals; and a vehicle body side rotation position estimation mechanism that estimates the rotational position (i.e., number of teeth) at the time of transmission by the transmitters, on the basis of the reception information for the wireless signal from the transmitters and the rotation position information for the wheels input via the communications line.