A frequency range of measurement is divided into a plurality of regions. For each region a test signal suitable for measurement of the region is generated independently, and the test signal is supplied to a device under test. The output of the device under test is converted by an AD converter into a digital signal, which is then subjected to fast Fourier transform. For each region the sensitivity of the AD converter is set for optimal conversion of the signal level at its input side by the converter, and then the fast Fourier transform is carried out. On the basis of the sensitivity set for one of the regions, the magnitude of the fast Fourier transform output in each of the other regions is corrected according to the sensitivities set for the other regions.