A tire air pressure monitor device adapted to monitor air pressure in tires, the tire air pressure monitor device comprising: a tire air pressure detection means installed on wheels of the tires for detecting the air pressure in the tires; a transmitter provided on each of the wheels for transmitting via wireless signal the air pressure along with identification information inherent to the transmitter; a receiver provided on a vehicle body for receiving the wireless signal; a rotation position detection means (4a, 4a', 11a, 12a) provided on the vehicle body to correspond to each of the wheels for detecting a rotation position of the wheel; and a wheel position determination means (11, 12) that is adapted to accumulate as rotation position data for the wheels, the rotation position of the wheels acquired a plurality of times when a wireless signal is transmitted including a certain identification information; and that is adapted to determine wheel position corresponding to the rotation position data having a smallest degree of dispersion from among the rotation position data as the wheel position for the transmitter corresponding to the identification information, characterized in that the wheel position determination means (11, 12) is adapted to convert the rotation position of each of the wheels to a vector with a start point at an origin of a two dimensional plane and an end point at a point on a circumference of a unit circle; to calculate a scalar quantity of an average vector of vectors for the rotation position data as a dispersion characteristic value; and to determine a highest value among the dispersion characteristic values as having the smallest degree of dispersion. a tire air pressure detection means installed on wheels of the tires for detecting the air pressure in the tires; a transmitter provided on each of the wheels for transmitting via wireless signal the air pressure along with identification information inherent to the transmitter; a receiver provided on a vehicle body for receiving the wireless signal; a rotation position detection means (4a, 4a', 11a, 12a) provided on the vehicle body to correspond to each of the wheels for detecting a rotation position of the wheel; and a wheel position determination means (11, 12) that is adapted to accumulate as rotation position data for the wheels, the rotation position of the wheels acquired a plurality of times when a wireless signal is transmitted including a certain identification information; and that is adapted to determine wheel position corresponding to the rotation position data having a smallest degree of dispersion from among the rotation position data as the wheel position for the transmitter corresponding to the identification information, characterized in that the wheel position determination means (11, 12) is adapted to convert the rotation position of each of the wheels to a vector with a start point at an origin of a two dimensional plane and an end point at a point on a circumference of a unit circle; to calculate a scalar quantity of an average vector of vectors for the rotation position data as a dispersion characteristic value; and to determine a highest value among the dispersion characteristic values as having the smallest degree of dispersion.