A signal analyzer provides frequency domain data at different time and frequency resolution. First type frequency domain data is derived from time domain data of a signal under test by first type frames and displayed as a spectrogram. A selecting box is displayed on the spectrogram for selecting the first type frames. The signal analyzer produces second type frequency domain data by treating the time domain data corresponding to the first type frequency domain data included in the selected first type frames with the selecting box as one frame by FFT calculation. The resultant second type frequency domain data are displayed as a spectrum that has different time and frequency resolution from the spectrogram.