To detect partial discharge economically and with high sensitiveness, by signal-processing the electromagnetic waves leaking out of the insulator in a container which accommodates a high-voltage electric apparatus in high insulative condition with a certain band width in a microwave region.

When partial discharge occurs inside a gas insulating switchgear 1, the electromagnetic waves by microwaves of tens of MHz to tens of GHz by partial discharge are propagated into inside of containers 2 and 3, etc. These electromagnetic waves leak through the cut of the cover 13 from the insulating spacer 4 caught by the flanges 2a and 3a of the containers 2 and 3. These leaked electromagnetic waves are led by a guide 12, and are received with the antenna 8 for microwaves of a microwave sensor 7, and are signal-processed by a signal processor 9. The processed received signal is sent to a signal processing judging unit 11, passing through a coaxial cable 10, and the existence of partial discharge, the size, etc., are judged from the signal level, etc., and the partial discharge detection is performed.