A tire deterioration inferring (estimating) method uses a tire deterioration inferring device placed on the inner surface of a tire. The method has: a deformation velocity measurement step of measuring a tire deformation velocity, which is the deformation velocity of the tire while the tire is rotating, and obtaining time-series changes in the tire deformation velocity; a calculation step of calculating peak values of the tire deformation velocity from the time-series changes in the tire deformation velocity; and an inference step of inferring the degree of the deterioration of the tire by using a first peak value, which is a peak value of the tire deformation velocity before and at the stepping of the tire or at and after the kicking of the tire, the first peak value being in the time-series changes in the tire deformation velocity.