PURPOSE: To realize the measurement for the input signals from the low to high levels by compensating the level reduction in the detection of the semi-peak value via the auxiliary amplifier.

CONSTITUTION: In case detection properties setting catch 21 is switched toward mean value detection M, switch 24 is connected to variable gain amplifier 17 and thus can be operated up to the maximum allowance level at each of the steps before detector 18. When measuring the noise field intensity, catch 21 is switched toward semi-peak value detection Q. And switch 24 is already switched to the output side of auxiliary amplifier 23. As a result, the reduction of the output level due to the detection of the semi-peak value can be compensated by amplifier 23 to eliminate the fear for the saturation of each step at the front step. Thus the operation can be secured up to the maximum allowance level.