PURPOSE: To surely hold a chip without the need for a detaching tool by forming a square groove storing a plate like chip freely slidably in parallel almost to a shank and the bottom face thereof at an obtuse angle for the feeding direction and projecting a spherical body to the side wall of the square groove.

CONSTITUTION: A main component force F1 and feed component force F3 act in the direction of pushing a chip 5 to the obliquely lower rear part for a feeding direction X, the direction of this force almost coincides with the direction of pushing the chip 5 to the bottom face 33a of a square groove 33, only the force in a plate thickness direction almost acts on the chip 5 and the assistance of the holding power by a clamp mechanism is not needed. The force sliding the chip 5 in the vertical direction is received by side walls 33b, 33c and a cutting resistance presses the chip 5 to the side wall 33c and the position of the chip 5 during cutting is stabilized. A backward force F2 pushes the chip 5 to the back part along the square groove 33 but is received by the back wall face of the square groove 33, the chip 5 is always pushed in as well and can be self-held. Consequently the force holding the chip 5 may be small, the compression length of a spring 43 is changed by adjusting the screwing depth of a pin 44, the pressing force of a steel ball 42 is adjusted and can be held.