PURPOSE: To attain ease of the adjustment of a detection start electric field and a detection voltage sensitivity by outputting a voltage having a prescribed logarithmic characteristic to an input electric field level of a wide range.

CONSTITUTION: A signal from an input terminal 13 is amplified and outputted by an intermediate amplifier 7, the signal from input/output terminals 13, 14 is rectified and added by the 1st and 2nd rectifier circuits 30, 31 and outputted from an output terminal 15. Since the negative terminal 32 of the 1st rectifier circuit 30 is connected to earth in order to take the level of the terminal 32 as a reference potential and the negative terminal 34 of the 2nd rectifier circuit 31 is connected to the positive terminal 33 of the 1st rectifier circuit 30, a sum voltage appears between the positive terminal 35 and the negative terminal 32. Thus, when no input is given to the input terminal 13, the potential of the terminal 35 reaches the ground potential and as the intermediate frequency to the input terminal 13 increases, the potential at the terminal 35 is increased.