An expansion shaft includes a first and a second shaft section aligned on the same axis. The second shaft section is inserted in a fit hole of the first shaft section while both the shaft sections are interconnected via a spline or serration. A resilient member is provided which possesses a single snap ring of C-shape fitted on a thin shaft portion of the second shaft section. An arm is axially extended from the snap ring. The arm resiliently presses against an inner circumferential surface of the fit hole for preventing the shaft sections from radially vibrating relative to each other.