PURPOSE: To make temp. compensation of a light receiving element with a relatively simple structure without using a temp. sensor by using a light receiving element for compensation to a device for detecting a fluid mixing ratio having a light transparent body disposed with a light emitting element and the light receiving element at both ends.

CONSTITUTION: The output from the light receiving element 36 for compensation is applied through a 1st operational amplifier 270 to a negative terminal of a 2nd operational amplifier 271 and the energization current to the light receiving element 35 increases so as to compensate a decrease in the light emission efficiency of the light emitting element 35 with an increase in the ambient temp. Since the refractive index of the fluid to be measured increases simultaneously, the output of the light receiving element 35 tends to increase; however, the output voltage from the 2nd operational amplifier 271 is gradually decreased by the drop of the output voltage of the light emitting element and, therefore, the quantity of the light emitted by the light receiving element 35 declines. The quantity of the light received by the light receiving element 55 is thus gradually decreased and the output thereof is returned to the normal, by which the compensation of the temp. is executed.