To improve estimation accuracy and reliability, when estimating a forward/backward force Fx and an upward/downward force Fz caused by the tire distortions at a sidewall part measured by a distortion sensor.

A method includes a tire distortion measurement step of obtaining N-sensor outputs t, by simultaneously measuring the tire distortion by N-distortion sensors 10 at a predetermined tire rotation phase angle position Q, an operation step to operate and determine the estimation value Fx0 of the forward/backward force Fx and the estimated value Fz0 of the upward/downward force Fz from estimation formula 1, by using the four sensor outputs t1-t4 among the N sensor outputs t.