PURPOSE: To increase a roundness remarkably by providing a detecting means to detect a deviation between a radius of a machined work and a target radius at specified intervals, a calculation means to calculate a tool locus interpolatedly, and a control means to rotate a tool along the calculated locus.

CONSTITUTION: In a cylindrically machining device in which a tool is rotated along a locus (b) of the center of a cutter to the outer periphery of a work WK to machine it cylindrically so that the radius of the work WK becomes a target radius, a detecting means which divides the outer periphery of the work WK at specified intervals, for example, into twenty four sections every 15° and detects a deviation between the radius of a machined work WK and the target radius every 15° is provided. Then, based on the deviations detected every 15°, the position of the tool is compensated every rotational angle of 15° of the tool and, based on the tool positions compensated every 15°, the locus of the center of the tool is calculated interpolatedly by a calculation means and the tool is rotated along the calculated locus by a control means.