PURPOSE: To improve cutting efficiency by preventing omission of a chip, by a method wherein a piece in a projected state to be fitted in a groove of the chip is formed on an end face of a shunk and at least the front of the piece in the projected state and that of the groove are stuck to each other to a rotary direction of the shunk.

CONSTITUTION: A form of a chip 7 possesses a width comprised of an inner and outer circum ferential surfaces 10, 11 and thicker than a plate thickness of a shunk. The inner and outer circumferential surfaces 10, 11 possesses respectively an inner circumferential groove 12 and two outer circumferential grooves 13. The grooves 12, 13 are extended to the top 15 from a bottom wall 14 of the chip 7 and comprised of the front 16 positioning in front to the rotary direction of a core bit 1 and the rear 17 positioning in rear to the same and the side 18 positioning in a widthwise direction of the chip 7 and a sectional form is in about a trapezoidal state. The chip 7 is fitted to the shunk 2 on an opening end face side of the shunk 2, a bottom wall 14 of the chip 7 is stuck to an opening surface 5 of the shunk 2 with silver soldering the side 18 and front 16 of the inner circumferential groove 12 of the chip 7 are stuck respectively to the outside 9 and a front end 19 of the piece 6 in a projected state. Similarly, the side 18 and front 16 of the outer circumferential groove 13 are stuck respectively to the inside 8 and front end 19 of the piece 6 in the projected state.