Patent ID: 8299909

Claim:
An apparatus for detecting a decrease in a tire air pressure, comprising: a rotation velocity information detection means for regularly detecting tire rotation velocity information regarding the respective wheels of a vehicle; a resonance frequency estimation means for estimating, based on the rotation velocity information obtained by the rotation velocity information detection means, a resonance frequency of the rotation velocity information; and a determination means for determining, based on the estimated resonance frequency, a decrease in the tire air pressure; wherein the apparatus includes a data rejecting means for rejecting data of rotation velocity information hindering the estimation of the resonance frequency; and wherein the data rejecting means is configured so that, when the rotation velocity information is cut to have a predetermined time length and the cut rotation velocity information is analyzed and when a predetermined frequency domain including the resonance frequency does not include a resonance peak or includes two or more resonance peaks, the data rejecting means rejects the cut rotation velocity information.