PURPOSE: To provide structure wherein reliable positioning of a shaft is performed, a work is improved, and quality is stabilized.

CONSTITUTION: A retaining ring for shaft mounting device comprises mainly a guide member 2 with a stock part 1 wherein a retaining ring SP is movably fitted in and a plurality of the retaining rings are arranged in an aligning state and is formed such that the retaining ring SP is pressed in the groove part of a shaft F on the work side through an opening and mounted in the groove part of a shaft F on the work side. The retaining ring for shaft mounting device is provided with a stopper 7 fixed on the tip side of the guide member 2 and having a support surface which holds the retaining ring dislocated from the tip of the stock part 1, a press piece 5 which pushes out the held retaining ring in a direction approximately paralleling the support surface of the stopper 7, an arm 4 which fixes the press piece 5 and is rotated against the energizing force of a spring means 3 to move the press piece, and a positioning part which is in arranged axial direction of the guide member 2 and causes the groove part of a shaft part to be positioned facing the retaining ring held on the support surface of the stopper 7.