PURPOSE: To prevent the change of a texture due to heating and then cooling following machining by forming a transfer area between a main cutter and a sub-cutter of a cutter plate into a curved shape where an end area contacts the extension of the sub-cutter and is continuously curved.

CONSTITUTION: A cutter plate 11 rotated to a longitudinal axis 12 has a sub- cutter 13 extending almost parallel to the longitudinal axis 12 and a main cutter 14 arranged almost perpendicular to the longitudinal axis 12. A transfer area 15 is provided between the main cutter 14 and the sub-cutter 13. The transfer area 15 includes a continuous curve on the main cutter 14 having a linear portion, the curve being provided to be continued to the tangential direction of the sub-cutter 13. Such a curve is the function of a conical curve, part of a hyperbolic curve, part of a parabolic curve, part of a logarithmic curve or part of a natural logarithmic curve. The formation of a thin hardened layer during machining is thus effectively prevented.