PURPOSE: To accurately control the machining depth on an insufficiently leveled bed for work piece by detecting the starting point of drill machining through electrical contact of a machining tool and work piece.

CONSTITUTION: Apart from the power supply for drill machining, low voltage is impressed between a bed 3 and a drill head 4. In this state, machining operation begins under the order from NC control device 11. In the case of countersinking machining, for example, the contact of a machining tool 5 with work piece 2 lets low voltage electric current flow between 2,5, which operates a detector 10 to detect the starting point for machining. Thereafter, feeding upward and downward is controlled by a pulse-motor power, which ensures accurate countersinking machining.