The Faraday and the polar magnetooptical Kerr effects in undoped Bi12SiO20 (BSO) crystals have been investigated. The dependence of a turn angle of polarization plane on magnetic induction for both effects was established to be nonlinear. The results were interpreted within the framework of combined electromagnetogyration model. Electrogyration part of the contribution was due to the formation of the longitudinal internal electric field induced by optical spin orientation. It was shown that magnetism of nonshared electronic pairs of Bi–O hepthaedra made the contribution in the total gyration of BSO crystal.