To make an operating condition capable of being known of a machine, by providing a music selecting means selecting a desired music from in a performance information memory means corresponding to the condition of a machine tool such as generating abnormality and a music regeneration means regenerating this selected music.

An MIDI sound source 9 is provided, in which an MIDI performance data in conformity to an MIDI standard stored in an HDD19 to be loaded on an RAM3 by a CPU7 is read from the RAM3, converted into a low frequency analog voice signal and output to an audio amplifier 11. During work or in the case of generating abnormality, from in an MIDI performance data stored in the HDD19, a music in accordance with a state of a machine tool is selected, this selected music is regenerated by the MIDI sound source 9. In this way, the state of the machine tool, for instance, the content of a generated error can be judged by the music.