PURPOSE: To enable satisfactory discrimination of a sound as emitted when fine foreign matters collide one another from extraneous noises by detecting a collision sound within an equipment to determine a characteristic of the collision sound.

CONSTITUTION: Sounds as emitted when a conductive foreign matter 3i existing in a bus container 1 for a gas insulation switch gear and a foreign matter 30 existing outside the container 7 collide separately on a walls of the container 1 are detected with sound sensors 11a and 11b and sound sensors 11c and 11d set on an outer surface of the container 1. Sound signals detected with the sensors 11a-11d are amplified separately with amplifiers 12a-12d. Signals after the amplification of outputs of the sensors 11a and 11b and outputs of the sensors 11c and 11d are inputted into foreign matter identification index computing device 15 for computing a foreign matter identification index. The signals after the amplification of the outputs of the sensor 11a and 11b are inputted into a sound source distribution computing device 14 through a position evaluator 13. A fine foreign matter discriminator 16 identifies a collision sound due to the foreign matter 3i within the container 1 or a sound due to another cause from a foreign matter identification index as output of the computing device 15 and a sound source distribution as output of the computing device 14.