Optical activity and Faraday effect in bismuth germanium oxide (Bi12GeO20) were investigated in the spectral region 430.0-800.0 nm. It was found that the best fitted model of optical activity in Bi12GeO20 to the experimental results was the Drude model of molecule coupled oscillators. However the relation of optical activity with the crystal structure is not clear yet. Due to the quite strong Faraday effect, Bi12GeO20 could possibly be applied to some optical nonreciprocal or polarization devices.