A wave generator of a strain wave gearing has a rigid plug and a needle roller bearing mounted on the plug external peripheral surface. The plug external peripheral surface has a retainer engaging part for engaging with a retainer that would move in an axis line direction and for restricting the movement thereof along the axis line direction. The retainer has an outer ring engaging part for engaging with an outer ring that would move in the axis line direction and for restricting the movement thereof along the axis line direction. The movements of the retainer and the outer ring can be restricted without providing a separate member. A wave generator of a strain way gearing can be realized, which is provided with a roller bearing capable of restricting the movement of the outer ring with a simple structure.