A shaft assembly includes: a shaft 1; and a rolling bearing 3 for rotatably supporting the shaft 1. The shaft 1 has a hollow configuration and is provided with a different nature member 25, formed from a material different from that forming the shaft 1, in a hollow portion 24 thereof. Thus, the shaft assembly accomplishes torque reduction by reducing the frictional resistance of the bearing and is adapted to reduce vibrations and noises associated with the use of the rolling bearing.