PURPOSE: To eliminate the envelope variation of reproduced signals even if the reflectivity of a disk fluctuates so as to make jitter smaller and error rate lower by amplifying the output of a differential amplifier circuit depending on the output of a summing amplifier circuit and using the amplified output as the reproduced signals.

CONSTITUTION: The light quantity of polarized light is changed by means of a Kerr rotating angle and an AC voltage is taken out at a differential amplifier circuit 13. Moreover, the whole light quantity is taken out as the output of a summing amplifier circuit 14. When the reflectivity increases, the output of the circuit 14 increases proportionally to the whole light quantity because the whole light quantity increases. In addition, the AC voltage also increases proportionally to the light quantity. When the AC voltage and the output of the circuit 14 are respectively inputted as a numerator and denominator, the increment of the AC voltage proportional to the change in light quantity can be cancelled, and when the reflectivity decreases, the decrement of the AC voltage can be cancelled similarly. Therefore, the level of reproduced signals can be stabilized and envelope variation can be eliminated. As a result, the jitter can be made smaller and the error rate can be made lower.