PURPOSE: To maintain strength of a machining bar and to make the bar light in weight by fixing an elliptical bar body constituted of an outside body part and an inside body part at the outside of a center bar shaft and making a shape of an outside face of the inside body part different from that of a cylindrical inside face addendum envelope of the outside body part, in the machining bar for a flat plate material cutting mechanism.

CONSTITUTION: A hollow body 2 of a cutter block is constituted of integral type outside part 8 and inside part 9 and sections a shaft 10 together with journals and end flanges at both the ends. The outside part 8 has an inside face 12 and an outside face 11 and the inside part 9 forms each separate hollow part by an outside face 13 and two separate inside faces 14, 15. The outside face 13 of the inside part 9 occupies about 180° around the shaft 10 and is constituted of a curved partial circular arc surface 21, a flat surface 22 and a projected pitch circular arc transition surface 23 connecting both the surfaces. Thickness of a wall part 19 of the outside part 8 is made substantially uniform, maximum thickness of a wall part 20 of the inside part 9 is made equal to maximum thickness of the wall part 19 and minimum thickness is set smaller than minimum thickness of the wall part 19.