| 摘要 |
A wheel speed is detected by a wheel speed detecting device, and a wheel speed signal is outputted to a reference value setting device. At the reference value setting device, the wheel speed signal is corrected such that a center of an amplitude of the wheel speed signal becomes a zero reference, and the wheel speed signal is amplified by carrying out a predetermined processing. Further, a selecting device selects, of the wheel speed signals during a predetermined period of time, wheel speed signals whose S/N ratios are poor, which are not to be used. A frequency detecting device judges a tire air pressure state by an air pressure state judging device from a number of times an amplified wheel speed signal crosses the zero reference (a number of times a sign changes).
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