Apparatus and method for alarming decrease in tyre air pressure

An apparatus for alarming decrease in tyre air pressure which detects the decrease in internal pressure of the tyre based on wheel speed information obtained from wheels (FL, FR, RL, RR) of a vehicle and accordingly alarms a driver. The apparatus comprises: a wheel speed information detecting means (1), a memory means (2d) for storing the wheel speed information of the respective tyres, a calculating means (2a) for calculating a judged value based on vehicle velocity and wheel speed information, and a judging means for determining a decrease in tyre air pressure based on correlation of the vehicle velocity and an outer diameter of the tyre at the time of performing high-speed running. It is therefore possible to determine a decompression of a tyre based on the correlation also in the case where the judged value does not become large owing to increased influence of centrifugal force during high-speed running.