PURPOSE: To establish a measuring method for an electric field or magnetic field capable of measuring the DC component, which has been not practicable according to conventional techniques, through the use of an optical Pockels element or optical Faraday element.

CONSTITUTION: The light emitted by a light emitter element 1 is fed to a light diverger 2 to generate divergence, and one of the beams after divergence is not left passing the area to be measured for the purpose of controlling constant the light emission level of the element 1 but fed back to a light receiving element 4 in a light emission level control circuit 3. Another beam of the divergent light is fed to a photo-sensor 10 through an optical fiber 17. According to this method, the light fed to the photo-sensor 10 is free from influence of the electric field and magnetic field produced around the object to be measured A, the DC component of the output beam from photo-sensor 10 after the intensity modulation process is not likely to be canceled. which enables also DC measurement, and a measuring system for AC/DC can be accomplished.