An installation base is secured to part of outer peripheral surface of cylindrical container 1 which has electric conductor 2 arranged thereinside, and a case which stores a Faraday-effect element is detachably secured to the installation base. Faraday-effect element 10 having two optical fibers 24A, 24B of prescribed dimensions, to provide first and second optical paths 11A, 11B maintained in parallel with a predetermined interval therebetween and placed so as to intersect axial direction of electric conductor 2, and having mirror 12 which reflects linearly polarized light at one end surface of each optical fiber. Linearly polarized light from the same light source injected into each of the optical paths 11A, 11B, and current flowing in electric conductor 2 is measured base on the Faraday rotation angle of the linearly polarized light reflected and returned from mirror 12.