PURPOSE: To eliminate the influence of a magnetic fields in adjacent phase and to take a measurement with high precision by providing a conductor with a part where a current is flowed in the peripheral direction of the conductor like a coil part, and allowing a photomagnetic field sensor to detect the axial magnetic field of the conductor with maximum sensitivity.

CONSTITUTION: The cylindrical conductor 2 arranged in a grounding case 1 is provided with a spiral coil part 4 having the axis in the same direction with the conductor 2 as a fitting part for the photomagnetic field sensor. The photomagnetic field sensor 3 is provided at the hollow part in the coil part 4 so that maximum sensitivity is obtained in the axial direction of the conductor. Then, a current flows in the conductor 1 in its peripheral direction at the coil part 4 and the magnetic field produced by this current is in the axial direction of the conductor. Therefore, this photomagnetic field sensor which detects the magnetic field in the axial direction of the conductor with the maximum sensitivity is not affected by a magnetic field in adjacent phase perpendicular to the conductor axial direction and the precision is improved.