The Faraday effect of magnetic opal films is studied at the visible wavelength slightly detuned from the Bragg diffraction wavelength. At this wavelength, we can expect the reduced group velocity of light and the enhancement of the Faraday rotation. The magnetic opal was self-assembled from a suspension of magnetic-core/silica-shell spherical particles onto glass substrates using vertical deposition. Despite of the structural disorder in the thick opal film, the optical rotation was enhanced 70 times over that of the structure less bulk at 10 T. The thin film with the highest structural quality shows an enhancement factor of 175. Thus, structure-induced enhancement was explicitly demonstrated.