To further enhance swing preventive effect, since the swing preventive effect of a bit installing unit for installing a driver bit on the tip of a spindle is not conventionally sufficient, for example, in a screw-fastening tool called an impact driver.

An operation sleeve 11 is constituted so as to move even in the axis J direction in response to its rotary operation, and an interval between a bit installing hole 5a of the spindle 5 and a bit inserting hole 11c of the operation sleeve 11 is set larger than an unlocking position, and is constituted so as to hold a bit B by a support span L3 larger than usual.