A portion of a steering shaft 2 capable of being advanced and retracted defines a ball screw shaft 2a of a ball screw mechanism 13. A motor rotor 10 of an electrically driven motor 8 for providing a steering assist force is externally mounted on a rotary nut 14. Mounting of the motor rotor 10 onto the rotary nut 14 is carried out so that the both can be adjustably aligned. By way of example, the rotary nut 14 has an outer peripheral surface formed with an annular protrusion 33 of a arcuate longitudinal sectional shape, having a surface thereof formed with a plurality of axially extending surface indentations 33a arranged side by side in a direction circumferentially of the rotary nut 14.