Apparatus and method for detecting decrease in tire air pressure and program for detecting decrease in tire air pressure

A method for detecting a decrease in tire air pressure based on wheel rotation speed information obtained from tires attached to a four-wheel vehicle. The method includes: a wheel rotation speed information detection step for detecting wheel rotation speed information of the respective tires; a first storage step for storing the wheel rotation speed information; a decreased pressure determination step for determining, based on the wheel rotation speed information, a decrase in tire air pressure; a second storage step for storing a relation between a standard driving torque and a vehicle speed obtained in advance; a decreased pressure determination prohibition step for prohibiting the determination of a decrease in tire air pressure when a vehicle turning radius is lower than a predetermined threshold value determined by the relation, and a decreased pressure determination start step for starting the determination of a decrease in tire air pressure when the turning radius is equal to or higher than the predetermined threshold value.