To achieve the straight cutting when a bar for boring is long by arranging a hardened guide member in each of a plurality of slots, and arranging an elastic member in each slot between the hardened guide member and a bottom surface of the slot related thereto.

An elastic member is coupled with a bottom surface of each slot, and stored between side walls. Each hardened guide member 34 is extended outward from the slot, and provided with an arc-shaped surface approximately adaptable to a surface of a machined bore 20. The urging force generated by the elastic member is maintained in the concentric array with the bore 20. A cutting tool 14 maintains the concentric condition with the machined bore 20. The array guarantees that the cutting tool 14 continues the straight drilling in a center passage 18. The hardened guide member 34 is constantly in contact with the machined bore 20 to guarantee that the cutting tool 14 machines a straight hole.