A detection unit ( 8 ) is disposed for detecting the magnitudes of f₁ and f₂ frequency components of a composite signal which is passed through a center electrode cable ( 4 ). A detecting signal generating unit generates a detecting signal corresponding to a gap between a nozzle ( 5 ) and a workpiece ( 6 ) from the magnitudes of the f₁ and f₂ frequency components of the composite signal, which are detected by the detection unit ( 8 ). As a result, even if plasma occurs in the gap between the nozzle ( 5 ) and the workpiece ( 6 ), the gap can be detected with a high degree of precision.