Method, apparatus, and program for detecting decrease in tire air pressure

A method of detecting a decrease in tire air pressure based on wheel rotation information obtained from tires attached to a 4-wheel vehicle. The method includes: a step of calculating tire rotation information obtained from the respective wheel tires; a step of computing a first determination value showing a difference between the sums of the wheel rotation information of pairs of wheel tires on two diagonal lines and a second determination value showing a difference between the sum of the wheel rotation information of right wheel tires and the sum of the wheel rotation information of left wheel tires; a step of assuming the computed first and second determination values as points on orthogonal coordinates for which any one of the first determination value and the second determination value is an X axis and the other is a Y axis and converting the points to polar coordinates; a step of determining position of tire(s) having a decreased pressure based on the argument of the points on the polar coordinates; and a step of comparing the moving radii of the points on the polar coordinates with a predetermined threshold value to thereby determine a decrease in tire air pressure.