Tire position determination system

A tire position determination system includes a tire pressure detector (4) coupled to a tire (2). A tire position determination unit locates the tire pressure detector (4) and determines a mounting position of the tire (2). A gravitational component force detection unit (10) generates gravitational force information of the tire pressure detector (4). A characteristic value acquisition unit acquires the angle of the tire pressure detector (4) from the gravitational force information. The characteristic value acquisition unit (19, 20) acquires a first characteristic value of the tire pressure detector (4) when a drive source of the vehicle (1) is stopped and a second characteristic value of the tire pressure detector (4) when the drive source is started. A characteristic value comparator (21) compares the first and characteristic values. An operation control unit (22) determines whether the mounting position of the tire (2) has been changed based on the comparison and controls the tire pressure monitoring unit accordingly.