PURPOSE: To make it possible to detect the vacuum degree of a vacuum valve during the supply of a current, by using a coaxial cable as the output wire connected to a discharge detector and coaxially providing a conductor connecting an earthing electrode and a ground potential part to the outer periphery of said cable.

CONSTITUTION: A discharge detector 6 using an elastic wave piezoelectric sensor and an earthing electrode 7 close to said detector 6 are attached to the outer peripheral surface of an insulating cylinder 1 and the outlet wires of the detector 6 and the electrode 7 are connected to one terminal of a coaxial cable 9. The cable 9 is formed by coaxially arranging the core wire 10 coming to the output wire of the detector 6 and a shield 11 through an insulating layer 12 and further providing a shield 13 and an insulating layer 14 to the outer periphery of said insulating layer 12. When a vacuum degree is lowered during the supply of a current to a vacuum valve, discharge is generated between electrodes 4a, 4b and the electrode 7 and charged particles are impinged to the shield 5 or the cylinder 1 to generate minute collision sound. This collision sound is converted to an electric signal by the detector 6 while said electric signal is transmitted to a control apparatus through the core wire 10 and the shiel 11 of the cable 9 to perform display, alarm or the detachment of the vacuum breaker.