PURPOSE: To realize an FFT analyzer for enlarging a measurement band without increasing the sampling frequency of an A/D converter by adding a band-pass filter and then properly switching between the filter and an anti-aliasing filter.

CONSTITUTION: The analyzer for sampling a signal to be measured in synchronization with a sampling clock fs, performing A/D conversion, and then performing FFT is provided with an low-pass filter 12 for passing a signal whose frequency is fs/2 or less, a band-pass filter 15 for passing a signal whose frequency is n.fs/2 to (n+1) fs/2 (here, n is a positive integer), and a switch 16 for alternatively selecting the output of the low-pass filter 2 or that of the band-pass filter 15. Then, either filter is selected according to a measurement frequency region and a signal to be measured is passed and then sampling is performed.