PURPOSE: To suppress the increase of a machining time owing to an exchange time.

CONSTITUTION: Looking ahead of a machining program is executed at S4-S8 during a time in which a tool selection command is read during the stating of machining and each time exchange of a tool is executed. A tool code for a read tool selection command is outputted to a tool exchanging device 3 and preparation operation of exchange of a tool is started at S9. Meanwhile, a machining program is orderly executed by a CNC device 1 at S2, S3, and S11. Since preparation operation of exchange of a tool can be completed during a time in which a tool selection command is read as execution statement, there is no need to re-start the machining program after waiting for take-out of a drilling tool and its movement to a tool exchange position like a conventional method, and a machining time of the whole of one machining program can be sharply saved.