PURPOSE: To expand a dynamic range which can be analyzed by a spectrum analyzer for which FFT is adopted.

CONSTITUTION: In a spectrum analyzer for which FFT is adopted, a mixer to which time-domain signal data to be analyzed and a local oscillation output of a local oscillator are inputted is provided, and the local oscillator has a construction wherein the local oscillation frequency thereof is shifted by some prescribed width. Moreover, a device for monitoring the original input level of the time-domain signal data to be analyzed, an adjustable-gain amplifier of which the gain is so adjusted as to correspond to the original input level and a display unit displaying the data after conversion into frequency-domain data are provided, and the display unit is equipped with an attenuator which offsets the gain adjustment of the adjustable-gain amplifier.