To speedily easily perform a check and correction of a machining program, and to perform a large portion of them even if using a personal computer, etc.

The machining program is simulated by one block while performing a graphic display of machining state. An interference check and an abnormal approach check are carried out. The detected interference block and abnormal approach block are stored. In an edit display, the stored interference block and abnormal approach block are displayed by discriminating them from other block. Upon correcting the program, the block is stored as a change block. By using simulation results, an operating state is switched prior to execution of the stored block in a check mode operation of an NC (numerically controlled) machine tool.