In an optical current transformer in which the Faraday rotation of light in a waveguide (2) enclosing the current is measured, the waveguide (2) is designed as an integrated-optics single-mode waveguide embedded in a substrate (1).Further elements of the measuring device, such as polarizers (13), mirrors (11) and Y couplers (12, 14), may likewise advantageously be integrated into the substrate (1).The integration leads to a sensor element (SE) which can be produced in a simple manner and is to a large extent insensitive to environmental influences such as temperature fluctuations and vibrations.