PURPOSE: To keep safety of an operator, to obtain productivity and to assure a feedback system in a component mounting machine having an NC control unit and a machine controller.

CONSTITUTION: An optical curtain 16 for detecting presence/absence of safety of an operation from an NC control unit 20 and a movable unit of a machine controller, a safety detector such as a mat switch 17, etc., the controller and the unit 20 are instantaneously stopped upon reception of a signal from the detector, and when the safety unit is released, it is recovered. Even if a feedback system of the controller 20 is defective, its operation is assured. Thus, presence/ absence of safety of the entire area of a front surface and a rear surface of a component mounting machine 14 can be detected, an NC motor 28 is instantaneously locked by a signal for detecting an unstable state, and stopped under conditions for restarting the controller. Further, a runaway due to trouble of the machine system can be completely prevented.