PURPOSE: To prevent chips from being sucked by providing a non-axial channel merging with an axial center channel for flush medium provided on a drill bit and opening it to a surrounded surface of the dill bit.

CONSTITUTION: A flush medium flowing to a drill bit 11 passes through an intermediate section 16a of a channel 16, and then enters into a recess 18 after passing through a branch channel 16c. Then, lubricant contained in the medium adheres to the internal surface of the recess 18, and enters gradually into a spline coupling 14 between a lower piece 11a of the bit 11 and a lowermost tube member 13a of a tube member group 13. Also, a pressure is uniformed in clearances between the channel 16 and the lower piece 11a of the bit 11 and the lower piece 11a of the bit 11 and the lowermost tube member 13a. In addition, the medium enters into the lower part of the channel 16 through a branch channel 16d, and flows to a front surface 19 of the bit 11.