PURPOSE: To enable accurate current detection without being affected by an electromagnetic noise at all by generating an induced current on a secondary side through a ferrite core with a primary current to be detected.

CONSTITUTION: When an AC current flows in a conductor 1, a magnetic field produced at the periphery of the conductor 1 forms a magnetic circuit along the ferrite core 2 and the secondary-side voltage is generated by variation in the current number of pieces of cross-linked magnetic flux of a winding part 4. When the secondary-side voltage reaches the value of the working voltage of a liquid crystal element 6, the element 6 turns on to orient liquid crystal molecules. Light which is emitted by a projecting element 8 and propagated in a 1st fiber 9a, on the other hand, is emitted from its one end to the element 6, but reflected by a reflecting plate 7 because of the oriented liquid crystal molecules and enters a 2nd fiber 9b, where it is propagated to be light-received by a photodetector 10. Therefore, when the element 6 is on, the element 10 receives the light from the element 8 to detect the current flowing through the conductor 1.