To provide a cutting tool life diagnostic apparatus which makes compatible reduction in the cost of a cutting tool and the maintaining of machining quality by accurately determining timings for replacing the cutting tool.

The cutting tool life diagnosic apparatus 10 diagnoses the life of the cutting tool 5 for machining a work 1 by obtaining driving force from a motor 8. The apparatus is provided with a motor electric energy detector 11 which measures an effective power waveform of the motor 8 during the machining of the work by the cutting tool 5 and a pattern recognition type output monitoring device 12 which takes in the effective power waveform of the motor during the machining of the work by a new cutting tool 5 as measured by the motor electric energy detector 11 as an initial waveform to set a reference effective power waveform for indicating the life of the cutting tool 5 while, after setting the reference effective power energy, the effective power waveform measured by the motor electric energy detector 11 is compared with the reference effective power waveform by the recognition of patterns to determine the life of the cutting tool 5.