A single-structured torque sensor using a magnetostrictive ring attached to the steering shaft of the steering-by-wire system of an automobile was developed. An (Fe_0.80Ga_0.15Al_0.05)₉₉Zr_0.5C_0.5 alloy, which exhibited the large magnetostriction of 134 ppm and the strong tensile stress of over 800 MPa, was used for the magnetostrictive torque ring. However, the sensitivity of the torque ring sensor was 0.12 × 10⁻⁴ T N⁻¹ m⁻¹, which is smaller than the 0.8 × 10⁻⁴ T N⁻¹ m⁻¹ needed for a steering-by-wire system. The ring was therefore heat-treated at 743 K under a compressed stress of 100 MPa via the spark plasma sintering method to increase the sensitivity of torque. As a result, the sensitivity of the torque ring sensor increased to 0.72 × 10⁻⁴ T N⁻¹ m⁻¹ although depending on the residual magnetic flux density.