Tire position determination system

A travelling state determination unit (21) of a tire pressure detector (4) determines a travelling state of a vehicle based on a gravitational component force detected by an acceleration sensor. An operation mode switching unit (22) of the tire pressure detector (4) switches the operation mode of the tire pressure detector (4) from a pressure determination mode to an automatic locating mode when the vehicle speed become lower than a threshold value. An interval of period transmission of radio waves in the automatic locating mode is set to be shorter than an interval of the periodic radio wave transmission in the pressure determination mode.