The invention provides a method for manufacturing a magnetostrictive torque sensor shaft (100), wherein the sensitivity of a sensor can be improved while the toughness is ensured and the lag error canbe reduced. Provided is a method for manufacturing a magnetostrictive torque sensor shaft mounting a sensor portion (2) of a magnetostrictive torque sensor (1). The method includes conducting heat treatment on a shaft material including chrome steel or chrome-molybdenum steel by carburizing, quenching and tempering, and conducting shot peening on the shaft material after the heat treatment at least on a position where the sensor portion (2) is to be mounted. The shot peening is conducted by firing shot with a particle size of not less than 0.6 mm and a Rockwell hardness of not less than 60 ata jet pressure of not less than 0.4 MPa for a jet exposure time of not less than 2 minutes.