A tire position determination system comprising: tire pressure detectors respectively attached to tires; and a receiver coupled to a vehicle body, wherein the receiver is configured to receive a tire pressure signal (Stp) from each tire pressure detector and monitor the pressure of the corresponding tire based on the tire pressure signal (Stp); wherein each tire pressure detector includes a unique detector ID, a gravity component detection unit that detects a certain directional component of gravity and generates a detection signal, and a transmission control unit (23, 24) that transmits a radio wave signal (Spk) including the detector ID based on the detection signal of the gravity component detection unit; characterized in that the receiver includes an interface that communicates with axle rotation amount sensors, wherein each axle rotation amount sensor detects an axle rotation amount of a corresponding axle, a tire mounting position determination processing unit (27, 28, 29, 30) that acquires the detector ID whenever receiving the radio wave signal (Spk) from each tire pressure detector and the axle rotation amount detected by each axle rotation amount sensor, associates the detector IDs with the axle rotation amounts, and determines mounting positions of the tires based on the axle rotation amounts associated with the detector IDs of the tire pressure detectors. tire pressure detectors respectively attached to tires; and a receiver coupled to a vehicle body, wherein the receiver is configured to receive a tire pressure signal (Stp) from each tire pressure detector and monitor the pressure of the corresponding tire based on the tire pressure signal (Stp); wherein each tire pressure detector includes a unique detector ID, a gravity component detection unit that detects a certain directional component of gravity and generates a detection signal, and a transmission control unit (23, 24) that transmits a radio wave signal (Spk) including the detector ID based on the detection signal of the gravity component detection unit; characterized in that the receiver includes an interface that communicates with axle rotation amount sensors, wherein each axle rotation amount sensor detects an axle rotation amount of a corresponding axle, a tire mounting position determination processing unit (27, 28, 29, 30) that acquires the detector ID whenever receiving the radio wave signal (Spk) from each tire pressure detector and the axle rotation amount detected by each axle rotation amount sensor, associates the detector IDs with the axle rotation amounts, and determines mounting positions of the tires based on the axle rotation amounts associated with the detector IDs of the tire pressure detectors. a unique detector ID, a gravity component detection unit that detects a certain directional component of gravity and generates a detection signal, and a transmission control unit (23, 24) that transmits a radio wave signal (Spk) including the detector ID based on the detection signal of the gravity component detection unit; an interface that communicates with axle rotation amount sensors, wherein each axle rotation amount sensor detects an axle rotation amount of a corresponding axle, a tire mounting position determination processing unit (27, 28, 29, 30) that acquires the detector ID whenever receiving the radio wave signal (Spk) from each tire pressure detector and the axle rotation amount detected by each axle rotation amount sensor, associates the detector IDs with the axle rotation amounts, and determines mounting positions of the tires based on the axle rotation amounts associated with the detector IDs of the tire pressure detectors.