BALL SCREW DEVICE AND METHOD FOR MANUFACTURING SCREW SHAFT

A ball screw device is achieved in which the number of components can be suppressed and the assembling efficiency can be improved even in a structure that rotationally drives a screw shaft via a planetary speed-reducing mechanism. A screw shaft 2 has a screw portion 10 having a spiral-shaped shaft-side ball screw groove 12 on an outer peripheral surface thereof, and a carrier 11 integrally formed with the screw portion 10 and constituting the planetary speed-reducing mechanism 7, and an inner ring raceway 14 constituting a rolling bearing 5 for supporting the carrier 11 so as to be able to rotate freely is directly formed on an outer peripheral surface of the carrier 11.