PURPOSE: To improve measurement precision by controlling an oscillation frequency so that light diffracted with a signal of known frequency to be superposed upon an electric signal to be measured is made incident as closely to a predetermined position on a photodetector array as possible.

CONSTITUTION: Two known signals supplied from an oscillator 7 are superposed by a multiplexer 6 upon the electric signal to be measured which is received by an antenna 5. The output signal of the multiplexer 6 is mixed with the output of a local oscillator 9 by a mixer 8 and converted to an intermediate frequency, and the resulting signal is amplified by an amplifier 10 and applied to a reed screen type converter 3. On the other hand, light 12 from a semiconductor laser 11 which is coupled with an end surface of a substrate 1 is made into parallel light by a lens 13 and then diffracted with a surface acoustic wave signal 4. The diffracted light 14 is converged by a lens 15 and focused on the photodetector array 16 provided on the end surface of the substrate 1.