PURPOSE: To perform both accurate storage and read-out of information with just a single laser diode by providing a control circuit which controls the driving current of the laser diode based on the difference between the output signal sent from a sample holder and a prescribed reference signal.

CONSTITUTION: The output voltage of a sample holder 9 is applied to the reverse input terminal of a differential amplifier 4 via a resistor 13. The reference voltage Vref is applied to a reference voltage generating source 6. A resistor 12b is equal to a feedback resistance of the amplifier 4, and the voltage Vf is controlled in relation to a resistor 12a and then applied to the reverse input terminal of the amplifier 4. The amplifier 4 calculates (Vref-Vf), and this difference voltage is properly amplified and applied between the base and the emitter of a transistor 5 via a resistor 12c. The transistor 5 flows a current proportional to said difference voltage to a laser diode 1. A resistor 12d controls the driving current Ir of the laser diode Ir.