The rotational direction of a main shaft is stored in a tool data file memory (101b) for each tool. Data necessary for preparing an NC program are stored in an RAM (105) interactively using an operation board (108) or a graphic display device (102). Thereafter, a processor (103) prepares the NC program using the data stored in the RAM. When a tool to be used for a predetermined machining step is determined in preparing the NC program, a tool select instruction for selecting the tool and the rotational direction of the main shaft that fits to the selected tool are found from the tool data file memory (101b), and a main shaft rotation instruction is produced to turn the main shaft in said direction.