Method and apparatus for detecting decrease in tire air-pressure and program for judging decompression of tire

A method for detecting decrease in internal pressure of a tire mounted to a vehicle on the basis of rotational information obtained from the tire. The method includes the steps of: detecting the rotational information of the respective tires; storing the rotational information of the respective tires; calculating a front and rear wheel ratio which can be obtained on the basis of rotational information of both front wheels and rotational information of both rear wheels of the vehicle; calculating a driving wheel torque of the vehicle; and obtaining a value of the front and rear wheel ratio when the driving wheel torque is zero on the basis of a relationship between the front and rear wheel ratio and the driving wheel torque. It is possible to shorten the time for obtaining effective front and rear wheel ratios.