PURPOSE: To detect easily the height of a tool in relation to a workpiece without making the tool abut actually on the workpiece.

CONSTITUTION: A pick-up 54 is arranged freely in rising and falling to a head 28 capable of rising and falling and arranged below the tip of a tool 46. Relative rising motion of the pick-up 54 to the head 28 is detected by means of a sensor 70. A distance between the bottom end of the pick-up 54 and the tip of the tool 46 is measured prior to work processing and memorized in a controller. The head 28 is lowered for detecting by means of the sensor 70 that the pick-up 54 abuts on the workpiece. As the tip of the tool 46 reaches the upper end of the workpiece by lowering the head 28 by the distance, an engraving machining of predetermined depth is applied on the workpiece by means of the tool 46 by setting the position as an original standard of a longitudinal axis.