PURPOSE: To achieve a highly accurate measurement stably by winding an optical fiber for sensor around a conductor for a specified number of times at a position of a grounding potential side which is away from the conductor by an insulation distance and then making connection to a light transmission/reception part with the optical fiber directly.

CONSTITUTION: A cylindrical ground shield 20 is placed concentrically fully away from a conductor 2V and insulation. A magnetic field sensor 21 is constituted so that an optical fiber is wound at an outer periphery of the shield 20 for a specified number of times, its winding start and winding end pass through a hole 10, and it is directly connected to a light reception/transmission part 12 which is fixed to a support container 6. Direct current can be measured according to Faraday effect by using the magnetic field 21 which is formed at this optical fiber, thus enabling temperature fluctuation due to heat build-up of the conductor 2V to be reduced and influence of birefringence which the sensor 21 receives as well as mechanical strain of the optical fiber to be reduced. Since the sensor 21 and the light reception/emission part 12 are close each other, light-axis deviation between both and fluctuation of light axis due to convection of gas are small. Also, influence of other phase magnetic fields can be avoided according to the Ampere law.