PURPOSE: To achieve a higher accuracy or a smaller size of the equipment, by a method wherein a n LED is used as light source and the forward voltage is detected to remove measuring errors unavoidably associated with changes in the wavelength resulting from changes in the temperature of the LED through the correction of the detection output of a light receiving section.

CONSTITUTION: An LED 7 emits light with a driving section 8 comprising a constant current source and the radiation light thereof is introduced into a light receiving section 4 through material 3 which varies the quantity of transmission light as function of the measured value. On the other hand, a voltage detecting means 9 is connected to the LED 7 to detect the terminal voltage and the output of the voltage detecting means 9 and the output of the light receiving section 4 are inputted into a signal processing section 5. With the signal processing section 5, changes in the wavelength of light emitted of the LED 7 associated with changes in the temperature are calculated by an output signal of the voltage detecting means 9 and based on the results, the signal from the light receiving section 4 is corrected to show the measured value on a display section 6.