PURPOSE: To attain a logarithm transformation with excellent liniarity as a whole by offsetting the increment/decrement of the output level of a logarithm circuit caused by passing an external input signal through a variable gain circuit.

CONSTITUTION: The level of an external input signal Vin is decided by a level deciding circuit 9, and the external input signal is inputted through a variable gain circuit 5, whose gain is changed according to a decision output, to a logarithm circuit 6, and a corrected voltage, which is changed according to the gain of the variable gain circuit 5, is outputted from a corrected voltage generating circuit 7. By subtracting or adding the corrected voltage from/to the output signal of the logarithm circuit 6, an external output signal is obtained. Consequently the increment/decrement of the output level of the logarithm circuit 6 caused by passing the external input signal Vin through the variable gain circuit 5 is offset. Thus the logarithm transformation at the excellent linearity is attained as a whole.