A wave generator of a strain wave gearing has a plug, a wave generator bearing mounted to an outer peripheral surface of the plug, an input shaft protruding from the center of the plug to a first side running along a center axis, and a bearing arrangement on which a support bearing supporting the input shaft is arranged. Relative to an outer ring end surface of the wave generator bearing that faces the first side, a center-side plug end surface of a plug end surface facing the same direction is in a position withdrawn from the first side toward the opposite direction. The bearing arrangement is provided between the center-side plug end surface and the outer ring end surface encircling the input shaft. The input shaft can be fitted within the width dimension of the wave generator, and a strain wave gearing having a short axial length can be achieved.