PURPOSE: To obtain an output voltage which is linear to an input voltage by driving a transistor(TR) which supplies a current to a light emitting element by a constant-voltage driving circuit with a constant voltage, converting a current which flows through a light receiving element into a voltage which is proportional to the logarithm of the current, and also performing reverse logarithmic conversion.

CONSTITUTION: When the DC component α.IF(DC) of a photocurrent IP varies owing to variation in current transmissivity α, etc., and the DC potential at a point B varies, a collector current Ic also varies correspondingly. Then a differential amplifier 5 suppresses the variation of the collector current Ic and performs negative feedback control over the potential at the point B so as to hold the minus input terminal and plus input terminal at the same potential. Therefore, when the voltage drop across an integration circuit 4 is reduced to '0' the voltage VR7(DC) across a resistance R7 becomes equal to a reference voltage Vref2. Consequently, the collector current Ic of a TR Q14 is equal to Vref2/R7 and comes to constant. Consequently, the gain Av of a reverse logarithmic amplifier 6 is held constant by holding the reference voltage Vref2 even if the photocurrent IP varies.