PURPOSE: To shorten a cutting time by providing a tool holder so as to be furnished regulatably upward and downward, and providing the protruding amounts of the succeeding cutting tools toward the direction of a plate material larger than that of the preceding cutting tool advance-wise toward the direction of an X axis.

CONSTITUTION: When a V-groove is formed at a plate material W, protrusion is made toward the down side of the rear side so that the succeeding bites may cut deeper than the preceding bite 63A. For that purpose, a bite holder 65 is made to be engaged movably upward and downward in an engagement recessed portion 67 formed at the lower part of a processing head 35, and an inclined engagement groove 69 is formed at the upper part of the bite holder 65, and an engagement with a guide portion 73 is made. And, a servo motor 79 is rotated, and the bite holder 65 moves upward and downward as the result of a wedge member 71 moving in the direction of an X axis, and the upper and lower positions are regulated. As a result, a deep groove processing can be done with one stroke, and a cutting time can be shortened.