A detection method for at least one of acting forces on tire among a back-and-forth directional force, a lateral force, vertical force, and a moment about a tire axis, detects the forces by strain outputs of strain sensors, which are attached on the tire and measure the strains of a sidewall portion. The method includes a strain measuring step to measure the strain of the sidewall portion with each of the above-mentioned strain sensors simultaneously at the rotational position P based on the tire rotational standard position X and to obtain the strain output per each strain sensor, and an acting force calculating step to calculate the above-mentioned force acting on the tire based on the strain output per strain sensor obtained on this strain measuring step.