PURPOSE: To obtain good cutting performance by holding balance in cutting by such a construction that No.1 throw-away tip is positioned approx. in the middle on the plane surface side of cutting chip exhaust gutter in a boring tool while No.2 throw-away tip is positioned at the periphery.

CONSTITUTION: No.1 throw-away tips 6, 7 are fixed on the face 5 of cutting chip exhaust gutters 3, 4 which screws 10. The tip 6 made from triangle plate is provided with a chamber 11 while the tip 7 is made from square plate. The outward release angle β of tip 6 is equal to or a little larger than the inward release angle γ and the release angle α of tip 6 is smaller than γ. By such an arrangement the out-of-balance in cutting due to centrifugal force is improved to ensure good cutting performance.