Apparatus and method for alarming decrease in tyre air pressure

An apparatus for alarming decrease in tyre air-pressure on the basis of a rotational information obtained by tyres attached to a four-wheeled vehicle, comprising rotational information detecting means; memory means for storing the rotational information; calculation processing means for calculating a reciprocal number of a turning radius and calculating a judged value, first judging means for comparing the judged value with a reference value at every region of the turning radius; and second judging means for cancelling a judgement of decrease of internal pressure when a difference between the judged values in the right and left turning regions is different from a difference between reference judged values in the right and left turning regions which are obtained at a time of initialising run.