Abstract: According to the tire air-pressure reduction detecting apparatus and method of the present invention, when a vehicle travels, the detected rotational angular speeds F.sub.i of the tires W.sub.i are multiplied by correction coefficients C.sub.i to correct initial differences. Vehicle speed V.sub.0 is calculated based on angular speeds F.sub.i0, and is put in a relationship formula f(V) to obtain a front/rear wheel ration f(V.sub.0) corresponding to the speed V.sub.0. Based on the front/rear wheel ratio f(V.sub.1) corresponding to a speed V.sub.1 at which the correction coefficients C.sub.i have been obtained, and the ration f(V.sub.0), there is obtained a variation of the front/rear wheel ratio f(V.sub.0)/f(V.sub.1), by which rotational angular speeds F.sub.3, F.sub.4 are then multiplied. The products are used for detecting a reduction in the air pressure of a tire W.sub.i, thereby to calculate a judgement value D. When D satisfies the conditions:D<-D.sub.TH1 or D>D.sub.