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A new high-sensitive magneto-optic readout technique, which allows detection of a relatively small magneto-optic rotation angle, is proposed. This technique essentially consists of a polarization modulation method ( which allows high-sensitive readout ) and a differential method ( which allows high-speed readout ). Applying this technique to a Co-Cr film ( Cr∼ 21.5 at. % ) with perpendicular anisotropy, Kerr rotation as small as 4.3' was detected at a signal-to-noise ratio (SNR) of about 34 dB in a 1 MHz bandwidth using a 25 mW He-Ne laser (λ = 632.8 nm ). The SNR was improved 17 dB compared to that obtained by conventional DC method. This technique is promising in the high-sensitive read-out with Co-Cr film.