PURPOSE: To realize the application of the photodetector in the optimum range and also ensure the working over a wide range of measurement without giving the range switching, by securing the continuous variation for the luminous intensity of the light source in accordance with the output of the photodetector.

CONSTITUTION: With increase of the turbidity of sample 1, the illumination, i.e., the optical current of phototransistor Q2 decreases along with increase of voltage U4. As voltage U4 is applied to the inverse input of comparator 4, the output level of comparator 4 lowers down. Accordingly, the current which is supplied to LEDQ1 through Q3 increases, and thus the luminous intensity increases. When the output of comparator 4 lowers less than the set value, Zener diode D2 conducts with the base bias of Q3 held at the level of the output of comparator 4. And the current supplied to Q1 becomes constant regardless of the level of voltage U4. Comparator 5 generates the output corresponding to the difference of voltage U1 and U2 generated at resistance R4 and R6 each. And this output turns to the output signal OUT showing the turbidity of sample 1.