Abstract: A chuck for fixing a rotary tool such a drill and an end mill to a machine tool is disclosed. This chuck comprises a main body comprising a taper shank for fixing the chuck to the machine tool and a tool support for clamping the rotary tool, a fastening ring for regulating fastening force by the tool support, a needle roller bearing interposed between the fastening ring and the tool support under an inclined state, and a gear mechanism for rotating the fastening ring. This gear mechanism comprises, for example, a worm wheel formed integrally with the fastening ring and a worm engaging with the worm wheel. When this structure is employed, the rotary tool can be fitted to and removed from the chuck without fixing a spindle of the machine tool, and the fastening ring can be turned without a great force.