PURPOSE: To insulate a measuring part completely and enable highly precise measurement with ease, by transduing a current to be measured flowing through a current measuring shunt into an optical signal and transmitting the same to the measuring part through an optical fiber.
CONSTITUTION: When a current I to be measured flows through a shunt 1, a current IL proportional to the current I flows through a light-emitting diode 2, which delivers an optical signal having the illuminance proportional to the current IL. This optical signal is transmitted to the light-receiving diode 6 of an optical signal receiving part B and transduced therein into a voltage signal proportional to the illuminance. Then, an amplifier 7 removes a voltage corresponding to a base current made to flow through the light-emitting diode 2 by supplying a bias voltage, and supplies a voltage signal proportional to the current to be measured to a measuring part 8. By this constitution, a current in a high- voltage part can be measured without being restricted by an insulation level and, in addition, the effect of noises can be eliminated.