Method for manufacturing steering shaft of steering device

A method for manufacturing a steering shaft of a steering device, the method comprising:

• a reduced-diameter section forming process in which a second-phase member is formed from a first-phase member being a metal rod, by forming a reduced-diameter section in the first-phase member;
• and
• a ball screw groove forming process in which a third-phase member is formed from the second-phase member by forming a ball screw groove in the second-phase member,
• wherein:
• the reduced-diameter section has a radius measured from a reference axis which is smaller than a material radius, wherein the reference axis is an axis that is parallel with a longitudinal direction of the first-phase member and extends through a center of the first-phase member at a cross section perpendicular to the longitudinal direction of the first-phase member, and wherein the material radius is a radius of an outer circumference of the first-phase member which is measured from the reference axis;
• in the reduced-diameter section forming process, the reduced-diameter section is formed in a first region that is a predetermined region including a first end of the first-phase member, wherein the first-phase member includes the first end and a second end in the longitudinal direction of the first-phase member;
• and
• in the ball screw groove forming process, the ball screw groove is formed in a second region of the second-phase member by rolling, wherein the second region is a predetermined region outside of the first region in a direction of the reference axis.