PURPOSE:To obtain a magnetic sensor small in size and high in reliability and capable of reading information recorded on a magnetic recording medium with high accuracy by utilizing effectively the principles of operation of a circulator. CONSTITUTION:A magnetic recording medium running section 16 in which a magnetic recording medium 15 directly contacts a Faraday rotation element 11 or runs with a very small gap to the element 11 is provided with a side opposite to the other side arranged with a magnet 14 of the Faraday rotation element 11. When a prescribed electromagnetic wave is inputted to an input waveguide 12 and a bias magnetic field of the magnet 14 is made constant at a prescribed strength, the prescribed electromagnetic wave is transmitted to an output waveguide 13 and outputted. When the magnetic recording medium 15 recorded magnetically is passed through the magnetic field, the synthesized magnetic field between the signal magnetic field and a bias magnetic field is activated to the Faraday rotation element 11 to control the rotation of the polarized plane to the electromagnetic wave. As a result, an output approximately proportional to the signal magnetic field appears in the output waveguide 13.