Method and apparatus for detecting decrease in tire-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 and storing rotational information of the respective tires; calculating and storing a slip rate; calculating and storing a correction coefficient for correcting a difference in effective rolling radii owing to initial differences between right and left front tires, a correction coefficient between right and left rear tires and a correction coefficient between front tires and rear tires; comparing a correction coefficient between front tires and rear tires with a correction coefficient between the front tires and the rear tires 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 correction coefficients and a specified threshold.