To make an output of a control signal optimum, even when a sensor is abnormally separated from a detection object, by making the output 0 when the difference between an electric signal corresponding to the distance between a sensor port and the detection object, and a specific electric signal, is more than the predetermined electric signal, and outputting an electric signal corresponding to the difference when the difference is more than the predetermined electric signal.

A signal Vi from an electrode of a pointed end of a torch 1, is input to a detecting circuit 4d, to generate a mating standoff signal Vs. A setting unit 4s generates a setting signal Vc corresponding to the desired mating standoff. Further an operating unit 4a outputs the difference between the signal Vs and the signal Vc (ΔV=Vs-Vc). The difference βV is transmitted to 1 terminal of a switch 4t which is switched by an output of a comparator 4c, and one of the input terminals of the comparator 4c. A power source output Vt is separately set for the other input terminal of the comparator 4c, for switching the switch 4t when ΔV>Vt. Thereby, an operating mechanism such as an actuator 3a can be prevented from the abnormal condition.