PURPOSE: To uniform the working depth from a plate upper surface by providing an interrupt control circuit for keeping the space between a copy sensor and the plate by driving a servo motor with the pulse generated by a pulse generating circuit.

CONSTITUTION: The longitudinal and lateral movements and rise and fall of an elevator base 22 are controlled by a NC control unit to automatically work a plate 1. A copy sensor 26 mounted on the lower surface of the elevator base 22 detects the space between the copy sensor 26 and the plate 1 upper surface and inputs it to a pulse generating circuit 28. The pulse generating circuit 28 generates a pulse corresponding to the detected space. When the space is changed, this is inputted to an interrupt control circuit 29 of an automatic controller 26 to drive a servo motor 19, and the vertical position of a tool 25 is automatically controlled at high precision following the undefined curved form of the plate 1 regardless of NC control.