PURPOSE: To reduce the cost of a main circuit part by IC-implementation by decreasing the number of used changeover switches to one by using one couple each of reverse logarithmic elements and output operational amplifiers and by providing two systems exclusively for recording and reproduction.

CONSTITUTION: A logarithmic element 31 is connected between the input and output terminals of an input operational amplifier 30, and the 1st output amplifying circuit 32 exclusively for a recording system and the 2nd output amplifying circuit 33 exclusively for a reproduction system are connected in parallel to the output side of said amplifier. The input terminal of the 1st circuit 32 is connected to the inverted input terminal of an output amplifier 35 via a reverse logarithmic element 34, and between its output terminal and the inverted input terminal, a filter 36 composed of resistances R, R and Re and capacitors C and Ce is connected. The input terminal of the 2nd circuit 33 is connected to the inverted input terminal of the output amplifier 38 via a reverse logarithmic element 37, and between its output terminal and the inverted input terminal, a resistance 2 is connected. A changeover switch S is changed over between a recording R and a reproduction P side, and connected to the inverted input terminal of an input amplifier 30 via the resistance 2R or filter 39.