A magnetostrictive torque sensor is provided that is capable of maintaining high detection accuracy, even when an excessive torque acts on a rotary shaft. The magnetostrictive torque sensor includes a magnetostrictive film 71 that is arranged on a second steering shaft 23, or a rotary shaft, having a substantially columnar shape so as to surround the second steering shaft 23 around its axis, and detects a rotational torque about the axis acting on the second steering shaft 23 based on a change in a magnetic property of the magnetostrictive film 71. Compressive stress remains on an outer circumferential surface of a sensor region 77 of the second steering shaft 23 around which the magnetostrictive film 71 is arranged. A plated layer of the magnetostrictive film 71 is arranged on the outer circumferential surface of the sensor region 77 in which compressive stress remains.