A wave generator for a strain wave gearing, which comprises an externally toothed gear and first and second internally toothed gears (2, 3), the wave generator configured for flexing an externally toothed gear into a non-circular shape and partially meshing with the first and second internally toothed gears (2, 3), and for causing meshing positions between the externally toothed gear and the first and second internally toothed gears (2, 3) to move in a circumferential direction, the wave generator (5, 5A) comprising: a rigid plug; a non-circular external peripheral surface formed on an external peripheral surface of the plug; first and second wave bearings (13, 14) press-fitted on the non-circular external peripheral surface; and an insertion member; the first and second wave bearings (13, 14) being disposed with a prescribed gap therebetween along a direction of a center axis of the plug; the insertion member (15, 15A) being disposed between the first and second wave bearings (13, 14) along the direction of the center axis; the insertion member (15, 15A) having an inner ring contact part and a retainer restraining part; the inner ring contact part being in contact with respective inner rings (16a, 19) of the first and second wave bearings (13, 14) from the direction of the center axis; the retainer restraining part facing respective ball retainers (17, 21) of the first and second wave bearings (13, 14) from the direction of the center axis; characterized in that the inner ring contact part is either integrally formed in the plug or fixed to the external peripheral surface of the plug so as to not move in the direction of the center axis; and the retainer restraining part is either integrally formed in the inner ring contact part or fixed to the inner ring contact part so as to not move in the direction of the center axis. a rigid plug; a non-circular external peripheral surface formed on an external peripheral surface of the plug; first and second wave bearings (13, 14) press-fitted on the non-circular external peripheral surface; and an insertion member; the first and second wave bearings (13, 14) being disposed with a prescribed gap therebetween along a direction of a center axis of the plug; the insertion member (15, 15A) being disposed between the first and second wave bearings (13, 14) along the direction of the center axis; the insertion member (15, 15A) having an inner ring contact part and a retainer restraining part; the inner ring contact part being in contact with respective inner rings (16a, 19) of the first and second wave bearings (13, 14) from the direction of the center axis; the retainer restraining part facing respective ball retainers (17, 21) of the first and second wave bearings (13, 14) from the direction of the center axis; a non-circular external peripheral surface formed on an external peripheral surface of the plug; first and second wave bearings (13, 14) press-fitted on the non-circular external peripheral surface; and an insertion member; the first and second wave bearings (13, 14) being disposed with a prescribed gap therebetween along a direction of a center axis of the plug; the insertion member (15, 15A) being disposed between the first and second wave bearings (13, 14) along the direction of the center axis; the insertion member (15, 15A) having an inner ring contact part and a retainer restraining part; the inner ring contact part being in contact with respective inner rings (16a, 19) of the first and second wave bearings (13, 14) from the direction of the center axis; the retainer restraining part facing respective ball retainers (17, 21) of the first and second wave bearings (13, 14) from the direction of the center axis; characterized in that the inner ring contact part is either integrally formed in the plug or fixed to the external peripheral surface of the plug so as to not move in the direction of the center axis; and the retainer restraining part is either integrally formed in the inner ring contact part or fixed to the inner ring contact part so as to not move in the direction of the center axis.