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

A method for detecting decrease in tire air-pressure in which decrease in internal pressure of a tire is detected on the basis of rotational information obtained from tires attached to a vehicle. The method includes the steps of: detecting rotational information of the respective tires; storing the rotational information of the respective tires; obtaining a slip rate between the tires and a road surface during running; comparing the slip rate with a slip rate which has been preliminarily obtained when at normal internal pressure; and judging simultaneous decompression of two wheel tires of both front wheels or simultaneous decompression of two wheel tires of both rear wheels on the basis of a relationship between a value obtained through comparison of the two slip rates and a specified threshold. It is possible to judge simultaneous decompression of two wheel tires of both front wheels or wheel two tires of both rear wheels.