Method and apparatus for detecting tire having decreased internal pressure, and program for detecting tire having decreased internal pressure

A method of detecting a tire having a decreased internal pressure by making a relative comparison among wheel speeds of tires attached to the respective wheels. The method includes a step of detecting wheel rotation information regarding the respective tires of the vehicle; a step of calculating wheel speeds based on the detected wheel rotation information; a step of calculating a decreased pressure determination value by relative comparison of the calculated wheel speeds; and a step of determining, when the resultant decreased pressure determination value exceeds a predetermined threshold value, that a tire has a decreased internal pressure. The method further includes a threshold value setting step of setting the predetermined threshold value based on, when assuming that a value of wheel speed comparison between a front wheel speed and a rear wheel speed is DEL2, (a) a relation obtained in advance, depending on a difference of the mass of the vehicle, between DEL2 and a load sensitivity during a vehicle turning, and (b) a load sensitivity obtained by a turning at the time of determination.