To monitor the abnormality of a tool even to the working of a non-mass production large component whose working components are different one by one and in which the working time for every component is longer than the service lifetime of the tool.

A sensor 14 is provided to the tool 13 of a machine tool for working the component 12. The abnormality of the tool 13 is monitored by comparing a working monitoring signal obtained by the sensor 14 with a working monitoring signal for reference. A tool 18, which is the same kind of tool as the tool 13, is used as a reference tool in order to obtain the working monitoring signal for reference regarding the component 12 whose working time is longer than the service lifetime of the tool 13. The working monitoring signal for reference is obtained by the reference tool 18 when the component 12 is worked with the tool 13.