To detect the magnetic field using polarization of a light, transparent magnetic material having a Faraday-effect is required. The Co-MgF₂ granular film is promising as a candidate of the Faraday element for the magnetic field sensing. In this study, Fabry-pérot resonant structure was introduced into the Co-MgF₂ granular film to enhance the Faraday-effect. It was found that the Faraday-effect per unit film-thickness [deg./µm] was enhanced due to a light localized in the Co-MgF₂ granular layer sandwiched by Bragg-mirror of the insulator multilayer. The maximum Faraday-effect of the proposed structure was -1.24 deg./µm, which was about 3.4 times enhancement compared with a single Co-MgF₂ film, when the number of layer period x in Fabry-pérot resonant multilayer was 5, where the structure was substrate/ (Ta₂O₅/SiO₂)^x/Co-MgF₂/(SiO₂/Ta₂O₅)²/Air. Furthermore, the multi cavity structure, consisting of the Fabry-pérot resonant multilayer film continuously placed, exhibited a 50% increase of Figure of merit (FOM) defined as Faraday-rotation angle per transmission loss [deg./dB].