PURPOSE: To enable accurate detection of a current to be measured, in a range from DC to a high frequency, without being affected by an external environment.

CONSTITUTION: A first polarizer 17, a Faraday element 18 for detection, a Faraday element 1 for modulation and a second polarizer 2 are disposed in this sequence on an optical axis 11 from the incidence side, and an external magnetic field (H) 27 in proportion to a current to be measured is impressed on the Faraday element 18 for detection. On the Faraday element 1 for modulation, a modulating periodic magnetic field (Hm) 3' which changes the dimension of the range of a change of a Faraday rotational angle in one period by nπ radians (n= l, 2, 3...) substantially and has a higher frequency than the external magnetic field (H) impressed on the Faraday element 18 for detection is impressed. The current to be measured is detected by subjecting a change in phase of the intensity of an emission light of the second polarizer 2 to a computation processing in a light computation processing part 19.