An apparatus for alarming decrease in tyre air-pressure in which decrease in internal pressure of tyres is alarmed based on rotational information obtained from tyres attached to a four-wheeled vehicle. The apparatus comprises a rotational information detecting means for detecting rotational information of each tyre, a memory means for storing the rotational information of each tyre, a calculating means for calculating reciprocals of turning radii based on rotational information of tyres attached to a driving shaft and judged values from the rotational information of each tyre, and a determining means for determining decrease in internal pressure from a relationship between the reciprocals of the turning radii and judged values. Determination of decompression can be reliable performed in the proximity of a region at which the lateral axis is O, since the judged values (DEL values) obtained while the differentiation is restricted are focused at points at which the lateral axis is O, that is, on the vertical axis.