To improve the sealability by forming a circular hole and a polygonal hole to agree with the shaft section of a tip tool to be inserted into a tip tool insertion hole of a dust-proof cap to prevent ingress of chips and dust from the outside.

A cylindrical dust-proof cap 7 formed of the elastic material is held at the outer circumference of the tip of a tool holder 6 by locking an annular locking projected part 7a to an annular groove 6b. The dust-proof cap 7 is prevented from its rotation in relation to the tool holder 6 by locking a projection 7b to a recessed part 6c. A circular hole 7d to agree with a shaft 1c of circular section of a tip tool 1, and a hexagonal hole 7e to agree with a shaft of hexagonal section of the tip tool are alternately formed on the inner circumferential surface of a tip tool insertion hole 7c on the front end side of the dustproof cap 7. Ingress of cup and dust inside the tip too holder from the outside, can be prevented and the sealability at the dust-proof cap tip tool insertion hole part can be improved.