PURPOSE: To eliminate the need for any filter circuit or tuning circuit and selectively detect only the light of a desired frequency by combining an amplifier having the transmission function of specific frequency characteristics, a photoelectric device of which the capacity changes, and a variable voltage power source.

CONSTITUTION: A silicon photocell 1 has a P type conductive layer 12 and an N type conductive layer 13 in conductive crystal via a flat surfacelike junction face 11, and is so constituted that when a reverse bias power source is connected via lead-in wires 14, 15 making ohmic contact with both conductive layers the electric current flows to the junction face 13 according to the light L incident to the face 10. The lead-in wire 14 is connected to the reflection input terminal 21 of an operational amplifier 2, and the lead-in wire 15 to the positive terminal of a variable voltage power source 3. The negative side of the power source 3 and the noninversion input terminal 22 of the amplifier 2 are grounded, and the output terminal 23 and the inversion input terminal 21 are connected via a resistance 4. The transmission function of the amplifier 2 is such that the phase delays 90° and the gain is inversely proportional to frequencies. This device is able to amplify and output only the incident light modulated by a desired frequency by regulating the voltage of the power source 3.