To perform finishing during the cutting while being vertically and horizontally fed.

Two cutting edge 6 and 6 connected to their respective nose sections 8 arranged in a pair of acute corner sections in the diagonal direction of an upper face 3 of a nearly rhombic plate like tip 1 are inclined in a projected curved line shape so as to approach a seat face as they are separated from the nose sections 8. The two cutting edge 6 and 6 have linear sub-cutting edges 6a and 6a which are orthogonal to each other near the nose sections 8. The rake face 9 of the upper face 3 has a square rake angle and is twisted and inclined as it approaches a seat face gradually along the cutting edges 6 toward the area of the diagonal line linking obtuse angle corner sections Q and Q. The tip 1 is secured to a tool body 21 with the nose sections 8 positioned on the external periphery of the tip. A cutting edge 6 becomes an external peripheral blade 6A, and its sub-cutting edge 6Aa is nearly parallel with the axis O of the tool body 21, and the other cutting edge 6 becomes a front blade 6B and its sub cutting edge 6Ba is arranged so as to be nearly orthogonal to the axis O.