To accurately measure a tread radius of a rotating tire. A method for measuring a tread radius has a tread radius measuring process which comprises a measurement step, an averaging step, and a calculation step. In the measurement step, the radial distances from three laser displacement meters to the tread surface of the rotating tire are measured to obtain radial distance data y1, y2, y3 whose number is m per the entire circumference of the tire. In the averaging step, an averaged value y1N, y2N, y3N is obtained by averaging the remaining radial distance data from which noise data have been removed by performing a smoothing process on the number m of the radial distance data y1, y2, y3. In the calculation step, a tread radius TR is calculated from the averaged values y1N, y2N, y3N and the distances x1, x2, x3 of the laser displacement meters in the axial direction of the tire.