1. A method for detecting a tire having a decreased air pressure based on wheel rotation information to be obtained from the tire attached to a vehicle in which there is a predetermined difference between a front wheel load and a rear wheel load, comprising: a step of acquiring the wheel rotation information to be obtained from the tire; a step of storing the wheel rotation information; a step of acquiring a load sensitivity of a tire dynamic loaded radius due to a load shift during a turning; a step of acquiring a speed dependency of the load sensitivity; a step of comparing a load sensitivity of a tire dynamic loaded radius at a speed acquired based on the speed dependency with preliminarily stored load sensitivities of dynamic loaded radiuses of various tires; a step of estimating a decreased pressure sensitivity of the tire dynamic loaded radius based on a result of the comparison; and a step of determining that the tire has a decreased air pressure based on a result of the estimation, wherein the speed dependency is acquired from a change of a front and rear wheel speed ratio.