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Faraday rotation is a well-known magneto-optical phenomenon: polarization plane of the light wave is rotated upon transmission through a magnetized medium. It is believed that most efficient Faraday rotators are transparent magnetic materials, such as rare-earth doped glasses and garnets, as well as diluted magnetic semiconductors, where magnetic ions create strong magnetization. In contrast, in conventional non-magnetic semiconductors the rotation due to magnetization of electron gas in the presence of equivalent magnetic field is much smaller. We show, that using optical orientation of electron gas in GaAs confined in a microcavity (Q=50000) it is possible to reach the rotation angles larger than 10° at only 1% spin polarization of the electron gas and in the absence of magnetic field ( Fig. 1 ). This strong rotation exceeds by orders of magnitude the rotation ever measured in bulk n-GaAs samples of similar concentration () [1] [2] . We deduce from these experiments the Verdet constant associated with the electron spin polarization density. This also allows for the direct detection of the extremely slow electron spin polarization decay (~250 ns) in the absence of the pump ( Fig. 2 ). It suggests that spin relaxation time in the dark can be much longer than the relaxation time extracted from Hanle depolarization in either photoluminescence [3] or Faraday rotation ( Fig. 1 ) experiments.