It is an object of the present invention to provide a rotor shaft capable of sufficiently securing a fastening strength between an inner shaft and an outer shaft, obtaining an excellent anti-rotation torque, and reducing a weight of the rotor shaft. A rotor shaft is divided into an inner shaft and an outer shaft. The inner shaft includes a tip end portion, a rear end portion, and a thick portion. The thick portion connects the tip end portion 41 and the rear end portion with each other in the radial direction, and has the same inner diameter as that of the tip end portion and the same outer diameter as that of the rear end portion. The outer shaft includes an inner cylindrical portion, a magnetic body-mounting outer cylindrical portion, and a flange that connects the inner cylindrical portion and the magnetic body-mounting outer cylindrical portion with each other. Outer peripheral surfaces and of the thick portion and the rear end portion and an inner peripheral surface of the inserting hole of the inner cylindrical portion are brought into contact with each other, and the inner shaft and the outer shaft are fastened and fixed to each other. The flange is disposed at a position opposed to the thick portion in the radial direction.