PURPOSE: To form a diagnostic device of cutting abnormality with excellent observational accuracy, by discriminating abnormality during cutting operation by the variable method through an abnormality discriminating method selector in accordance with the kind of a tool and the quality of a material to be cut.

CONSTITUTION: Load data during cutting operation of a work aiming at observation and reference data of a cutting load of a model work are fed to an abnormality discriminator 8 respectively from a load data input device 6 and a reference data input device 7. While an abnormality discriminating method selector 11 inputs the number of a tool and the quality number of a material to be cut respectively from a tool number input device 9 and a quality number input device 10, and outpus a switching instruction to the abnormality discriminator 8 by selectively taking out the abnormality discriminating method corresponding to said tool and quality numbers input to the selector 11. The discriminator 8, corresponding to the switching instruction output from the selector 11, switches the abnormality discriminating method and outputs a diagnostic result of abnormality by comparing the cutting load data input from the input device 6 with the reference data.