Anomalous Faraday rotations of the following three series of polycrystalline garnets weremeasured; 1) Gd3-xBixFe5O12, 2) Gd2Bi1Fe5-yGayO12 and Gd2Bi1Fe5-yInyO12, and 3) Y2.25Bi0.75Fe5-yGayO12 and Y2.25Bi0.75Fe5-yScyO12. In series 1 and 2, the rotations were measured in the visible region at room tentperature. The rotation of series 1 increased linearly with x up to x=1.4, and the magneto-optical figure of merit reached 23 deg (at 0.56 µm) in Gd2Bi1Fe5O12. It was found in series 2 that the further substitution of Ga for Fe up to y=1.0 slightly decreased the figure of merit. In series 3, the rotations were measured in the near infrared region at a low temperature (8K) and the results were analyzed by using a one-ton model.It was found that two transitions near 3.1 eV and 2.8 eV, which are related to octahedral irons and tetrahedral irons respectively, were responsible for the anomalous rotation. These transitions were tentatively assigned to oxygen-iron charge-transfer transitions.