PURPOSE: To detect a minute zero-phase current without disappearance in noises, by coupling a plurality of Faraday elements in series with polarization-plane preserved optical fibers, and synthesizing optical signals corresponding to currents flowing through power transmission lines.

CONSTITUTION: Light, which is emitted from a light emitting element 9, is inputted into a polarizer 11 through an optical fiber 10 and undergoes linear polarization. The light is passed through a Faraday element 4 and rotated by a Faraday angle corresponding to a current flowing through a power transmission line 1. The light is passed through a Faraday element 5 by way of a polarization-plane preserved optical fiber 7 and rotated by a Faraday angle corresponding to a current flowing through a power transmission line 2. The light is passed through a Faraday element 6 by way of a polarization-plane preserved optical fiber 8 and rotated by a Faraday angle corresponding to a current flowing through a power transmission line 3. The Faraday angle of the polarization plane is converted into the intensity of the light with an analyzer 14. The intensity of the light is transduced into an electric signal with a photodetector 12. The level is corrected with a signal processing circuit.