Apparatus and method for alarming decrease in tyre air pressure

An apparatus for alarming decrease in tyre air pressure comprising a rotational information detecting means for detecting rotation information of the respective tyres (Wi); a rotational information memory means; a judged value calculating means for calculating judged values based on the rotational information; a judged value memory means; a transition calculating means for calculating transition of the judged values based on the stored judged values; a judging means for determining a decrease in tyre air pressure; and an alarming means for issuing preliminary alarm prior to issuing a predetermined alarm for informing decreased in internal pressure in case the judged value tends to move to exceed a predetermined threshold. It is possible to appropriately inform the driver of the vehicle of a decompressed condition of a tyre to assist the driver to drive safely.