PURPOSE: To prevent the mixing of a pollutant by installing a base seal onto a base frame, making them cooperate with each other through a conical seal and a recess, and forming a space between the base seal and the recess and forming an air seal.

CONSTITUTION: A mechanism driving a shaft is set up into the base frame 18 of the rock drill, and the shaft is connected to a cone 10 and the cone 10 is turned and driven on a bearing surface 14. A lubricating device 26 and an annular through 27 are fitted to the spherical bearing surface 14, and a base seal 80 is attached onto the base frame 18. Surfaces 84, 86 with the inclined section 82 of the base seal 80 and the surfaces 58, 70 of a conical seal are formed in a spherical shape, and moved concentrically when a head is rotated. An air seal 119 is installed between the inclined section 82 and a cone supporting member 16. Accordingly, the infiltration of a pollutant can be prevented.