PURPOSE: To improve the monitoring accuracy without any troublesome adjustment by deciding concentration according to a detected value which is amplified and quantized and feeding the output value of an AD converter back to a DA converter digitally.

CONSTITUTION: The output of an optical sensor 3B for concentration measurement which detects light from a light source 1 through a sample 2 is amplified by a subtracting amplifier 8. Its output is quantized by the AD converter 9. A digital control part 11 decides the concentration according to the quantization output. Further, the output of the converter 9 is passed through the control part 11. A subtraction value is supplied to an amplifier 8 with the analog output of the DA converter 10 and thus fed back digitally. Then the analog output range of the converter 10 is set wider in conformity with the amplifier 8.