An expandable shaft includes an outer shaft and an inner shaft. Plural rolling elements are pinched elastically in a space between raceway grooves of the outer shaft and the inner shaft. An outer peripheral surface of the inner shaft includes flat portions parallel to each other. An inner peripheral surface of the outer shaft includes flat limiting portions parallel to each other. The respective limiting portions limit relative rotation of the outer shaft and the inner shaft by engaging with corresponding flat portions. A deformation promoting portion is placed in a specific region of the outer shaft between a plane including a center of curvature of the raceway groove of the outer shaft as well as a central axis line of the outer shaft and each limiting portion.