PURPOSE: To finish in a desired finishing face without removing a work from a specular work device, by providing a vibration meter which detects the vibration in the axial direction of a spindle and a computer device which calculates working conditions by the detection value of the vibration meter, preparing NC data and transmitting the NC data to an NC device.

CONSTITUTION: When the work material/shape are input to a computor device 1 first, the computor device 1 prepares NC data by selecting competent working conditions from the working conditions stored by the work material/shape. The work is then performed by transmitting the NC data on each one record to an NC device 2 by a transfer cable 17. At this time the vibration of a spindle 4 is measured by a vibration meter 15 and input to the computor device 1 via an A/D convertor 16. The computor device 1 discriminates as to whether or not the value of the amplitude thereof is within the allowable grange, discriminating whether or not the present working depth is at a desired value in the case of being within the allowable range, completing the work in case of being at a desired value and preparing the succeeding NC data in the case of NO.