To easily manufacture a magnetostrictive torque sensor with a high capacity.
The magnetostrictive torque sensor 1 comprises a shaft 2, a magnetostrictive material layer 3 and a detection device S for detecting a permeability of the magnetostrictive material layer 3. A material consisting of 2.5-12.0 wt.% of Mo, 2.0-5.5 wt.% of B and the balance of Ni and Fe of a weight ratio of 0.18-2.3 to Ni, including unavoidable impurities is used as the magnetostrictive material constituting the magnetostrictive material layer 3. An age hardening material having a melting point higher than that of the magnetostrictive material is used for the shaft 2. The shaft 2 and the magnetostrictive material are heated to a temperature of 1085±2°C for fusing, diffusing and bonding them. The shaft 2 diffusion-bonded with the magnetostrictive material is subjected to age-hardening treatment.