A first frequency analysis range and a second frequency analysis range narrower than the first one are set with an operation panel 34, etc. A first signal path 171 produces first time domain data of a frequency converted signal under test by a first data production rate depending on the first frequency analysis range. A second signal path 172 produces second time domain data of frequency converted signal under test by a second data production rate depending on the second frequency analysis range and slower than the first data production rate. A CPU receives the first and second time domain data in parallel and produces first and second frequency domain data by FFT wherein frequency shift amounts in the frequency conversions in the first and second signal paths are different depending on the difference between the center frequencies of the first and second frequency analysis ranges.