Abstract: Upper-and-lower cutouts (25a) and right-and-left cutouts are formed in a flange portion (23) of a holder main body (12). In a fore portion of the outer circumference of the flange portion (23), is provided a first operation ring (41), which is urged rearward by a compression spring (42). A taper portion (44) inwardly slanted as viewed toward the rear as well as a straight portion (45) are formed in the operation ring (41), and a drive key portion to be fitted to the key way of the spindle through the right-and-left cutouts are provided on both right and left sides of a rear face (45a) of the straight portion (45). In the rear of the operation ring (41), is provided a second operation ring (51) via two spacers (52).