PURPOSE: To output the detection voltage linearly proportional to the logarithmic value or an input signal voltage.

CONSTITUTION: A first differential amplifier 11 having first and second diodes D11 and D12 as the load and a constant current source 12 which determines the operation resistances of first and second diodes D11 and D12 are provided. A second differential amplifier 13 to which the output of the first differential amplifier 11 is supplied, a rectifying circuit A14 which rectifies an output V13 of the second differential amplifier 13, and control circuits A15 and A16 which perform the feedback control of the operation current of the second differential amplifier 13 in accordance with a rectification output V14 of the rectifying circuit A14 to control the mutual conductance are provided. AD input signal voltage V36 is supplied to the first differential amplifier 11. AD output voltage VRX of control circuits A15 and A16 is taken out as the detection output indicating the logarithmic value of the level of the input signal voltage V36.