PURPOSE: To make it possible to develop an NC program easily by increasing/ decreasing a distance preset in an NC device along a command arc of an NC program by means of both a judgment means and increasing/decreasing means of a control device according to an NC command.

CONSTITUTION: If presence of an input shift command in a block is judged at a shift command judgment means 71 of a program analysis means and further presence of a specified NC code in the same block is judged by a specified code judgment means 72, a straight line and arc distances preset in an NC device are increased/decreased along a locus of the shift command by means of a shift command increasing/decreasing means 73. Here for example, to interpolate an arc from a start point (u, v) to an old end point (a, b), a new end point (a', b') is obtained by a given distance decrement (c) along the arc locus and is transferred via a transfer means 74 to a machine control means. Thus, an NC program can be developed easily and work efficiency is improved.