Abstract: A magneto-optic characteristic measuring device which is adapted to measure a Kerr rotation angle is a kind of magneto-optic characteristic exhibited in materials used for magneto-optic recording media. Known materials exhibiting a magnetic Kerr effect have very small Kerr rotation angle ranging from about 0.1.degree. to about 0.5.degree.. As a result, it is difficult to accurately measure such a small Kerr rotation angle using known measuring devices due to an influence caused by noise components. In the magneto-optic characteristic measuring apparatus, its electronic circuit unit, which serves to magneto-optically modulate a laser beam from an optical unit introduced in a beam splitter via a polarizer and to record the modulated beam in an X-Y recorder using a detect signal from an optical detector unit, includes a lock-in amplifier,.thereby accurately measuring a small Kerr rotation angle.