PURPOSE: To dispense with the supply of a power source, by transmitting the light of a light emitting element to the outside of electric machinery as a voltage detection signal by an optical fiber.

CONSTITUTION: First and second potential parts and terminal parts 2, 3 of electric machinery 1 are respectively connected and a current flows to a circuit consisting of a current-limiting impedance 4 and a light emitting element 5 through the terminals 2, 3 by the voltages generated at both potential parts to allow the light emitting element to emit light. The light of the element 5 is transmitted to the outside of the machinery 1 by an optical fiber 6 and utilized as a voltage detection signal. This detection signal is led out to the outside of the high voltage machinery 1 as a light signal and, therefore, the insulating distance to a circumferential metal part becomes unnecessary and miniaturization is attained. Because the voltage difference between the terminals 2, 3 is used as a power source and potential difference therebetween is detected, the power source for a detection circuit is dispensed with. Further, because this apparatus comprising the light emitting element can be arranged in close vicinity to the high voltage machinery, an electric signal circuit can be shortened and electromagnetic induced noise is reduced and reliability is enhanced.