An apparatus for alarming decrease in tire air-pressure which alarms decrease of internal pressure of a tire on the basis of a rotational information obtained by tires attached to a four-wheeled vehicle. The apparatus comprises 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 regions of the turning radius; and second judging means for canceling 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 initializing run.