Tire pressure monitoring system

A vehicle C is provided with four wheel speed sensors VS (VSfl, VSfr, VSrl, VSrr) (wheel vibration detection means) for detecting wheel speed V (Vfl, Vfr, Vrl, Vrr) at respective four wheels W (left and right front wheels Wfl, Wfr and left and right rear wheels Wrl, Wrr). When the tire pressure is in the normal value, a tire pressure monitoring system prepares vibration frequency reference spectrum data for predetermined vehicle speed bands based on the detection values of each wheel speed sensor VS. The tire pressure monitoring system compares the current vibration frequency spectrum data and the vibration frequency reference spectrum data corresponding to the current vehicle speed band, and determines that the tire is underinflated if the comparison result exceeds a predetermined value.