A vehicle velocity detector 15 is provided, velocity V is detected from position data of the vehicle calculated by using signal from satellites, which is received by a GPS receiver 11 installed to a vehicle body, wheel rotation velocity Vw0 detected by a wheel velocity sensor 12 is corrected in accordance with tire inner pressure detected by a pressure sensor 13 so as to obtain wheel rotation velocity (correction value) Vw, velocity ratio R=(Vw/V), which is ratio of the corrected wheel rotation velocity Vw and the detected wheel velocity V, is calculated and tire wear amount is estimated in accordance with velocity ratio R so that tire wear amount can be measured precisely without processing the tire tread portion.