A method for alarming decrease in tire air-pressure, which detects decrease in internal pressure of a tire attached to a vehicle and accordingly issues alarm. The method includes the steps of: detecting rotational information of the respective tires; storing the rotational information of the respective tires; displaying a summer tire, a winter tire and a run-flat tire; designating a currently attached tire from among the displayed tires and switching to a threshold for judging decrease in internal pressure which matches the tire; judging decrease in internal pressure of a tire by using the threshold; and issuing an alarm on the basis of the judgment of the judging means. Since thresholds which match tires can be suitably switched when the driver operates the touch panel for discriminating presently mounted tires, it is possible to eliminate erroneous alarm and to perform judgment of decompression at even higher accuracy.