Abstract: A problem where a tool is deformed when a retaining ring is fitted on a shaft is solved. Plate members (2, 2a) positioned at at least one side of a full back surface of a retaining ring (W) are arranged at a holder (1) provided at the head of a tool body. A contact portion (3), a portion opposite an opening portion (Y) in the outer periphery of the retaining ring (W), is provided more backward than the heads of the plate members (2, 2a). A fall-off prevention structure (4) from which the retaining ring (W) can be easily detached is provided at the holder (1). There are sheltering spaces (5, 5a) arranged outside both side portions of the retaining ring (W) in its installed state. When the retaining ring (W) is installed to the holder (1), the retaining ring (W) does not fall off because of the fall-off prevention structure (4).