Abstract: In a tire air pressure estimation based upon vibration components extracted rom a vehicle wheel speed signal, when the vehicle is a driven vehicle wheel, the precision of estimation is improved by canceling the components of vibration due to the resonance of the drive system from the vibration components based upon which the estimation calculation is made. The vibration components due to the resonance of the drive system can be canceled by canceling those vibration components extracted from the vehicle wheel speed signal which are the same in the phase between left and right driven vehicle wheels. When the tire air pressure of one of a pair of left and right driven vehicle wheels is lower than that of the other, the lower tire air pressure is more correctly estimated. Therefore, the higher tire air pressure may be estimated according to the lower tire air pressure against the drive system resonance.