Abstract: In an optical current transformer, an optical sensor is made up of two optical sensor units which are independent of each other. Incident light beams are applied to the optical sensor units, and the emergent light beams from the latter are subjected to addition, to obtain a measurement signal which is proportional to a current under test which flows in a conductor. The optical sensor units are substantially U-shaped bars, which are Faraday effect glass bars which are rectangular in cross section and have 45.degree. reflecting surfaces at the corners.