PURPOSE: To simplify the circuit constitution without using the adder, the subtractor and others by using the variable resistance element for the resistance of the time constant circuit which decides the emphasis amount of the emphasis circuit and then giving the control to the resistance element in accordance with the level change.

CONSTITUTION: Compressor 28 of the narrow sense is constituted by voltage control-type amplifier 29 and level detector 31 which perform the opposite operation to each other, and then expander 44 of the narrow sense is formed with voltage control-type amplifier 42 and level detector 43. Then compressor 28 and expander 44 are connected via tape recorder 21. Furthermore, pre-emphasis circuit 28 and deemphasis circuit 50 are connected between input terminal 22 and compressor 28 as well as between output terminal 41 and expander 44 each. Thus a noise reduction system is realized for recorder 21. Photoconductive elements 26 and 55 are used for the resistance which is connected in parallel or in series to resistance 23 and 53 which decide the emphasis amount of emphasis circuits 28 and 50, and these resistance elements are formed so as to be controlled in accordance with the level change of detectors 31 and 43. Thus an effective noise reduction is ensured.