To minimize the operation stop time of a machine tool by preparing another file so as to be able to compute tools to be used without breaking the synchronization of a file that is synchronously fetching the working state of spare tools.

Tools used for machining according to unmachined NC data in machining during schedule operation are computed from the tool computed result and a virtual tool pot control file obtained in steps S1, S2 (step S3). In the case of confirming the insufficiency of tools by a judgment made in a step S4, if there is a shortage on the tools in want, around what time machining with the use of the tools judged to be in want is started with the present as a starting point, is displayed or printed in addition to information of the service time of the tools (step S5). The tools in want and required for a worker are informed in a list, screen display or the like (step S6).