TIRE AIR PRESSURE MONITOR DEVICE

When the difference between the first rotating period Tp determined based on detection by the G sensor 2b and the second rotating period Ta determined based on the detected value of the wheel velocity sensor 8 is equal to or smaller than the prescribed value ±, the rotational position of each wheel corresponding to the wireless signal transmitted at the prescribed rotational position is adopted in determining the wheel position; when the difference is over the prescribed value ±, the rotational position of each wheel corresponding to the wireless signal transmitted at the rotational position different from the rotational position is not used in determining the wheel position.