PURPOSE: To prevent the collision of a nozzle and failure in cutting in a mechanism for maintaining a specified relative distance in a Z direction between the X-Y direction scanner on a table and the nozzle by stopping the follow up of the nozzle in the Z direction when the detecting distance in the Z direction changes quickly by a specified value or above.

CONSTITUTION: A nozzle 2 is scanned in X-Y directions by a scanner 5 on a table 3, and the distance between the nozzle 2 and a member 8 to be cut is made constant by a position detector 6. The output of the detector 6 is made into a voltage by an amplifier circuit 12 and a delay circuit 13 and said voltage is put into a comparator 14. Said voltage is compared with the voltage after the specified time and when the difference deviates from he set value, a Z-axis control device 15 is stopped. The operation is resumed when the voltage is brought in the set value. The wrong movement of the nozzle in the Z direction when the detector 6 detects the cut hole, etc. is thus prevented.