For forming an annular magnetostrictive coat on an outer peripheral surface of a rotational shaft associated with a magnetostrictive torque sensor, a magnetostrictive-coat forming method comprises a step of fitting a cylindrical masking jig over the outer peripheral surface of the rotational shaft and securing the masking jig to the outer peripheral surface, a step of placing the rotational shaft in a plating tank to thereby form the magnetostrictive coat by plating on the outer peripheral surface, and a step of detaching the masking jig from the rotational shaft.