PURPOSE: To offer a feed control method by which high straightness can be maintained stably in deep hole machining by temporarily releasing all stress acting on a cutting tool.

CONSTITUTION: Step feed whereby cutting feed speed of a cutting tool changes by stages is performed, and, while the cutting tool is performing deep hole machining with its cutting feed speed set at normal feed speed (range II) in this step feed, its cutting feed speed is temporarily lowered to the stress removal feed speed (range III) at specified intervals, thereby all stress temporarily acting on the cutting tool is decreased, or released to zero, and the curve attitude of the cutting tool restores to its original state by its own elasticity during that time.