Constellation display allows a user to evaluate the quality of a signal under test with the constellation display even if the signal is a digital modulation signal that has many possible ideal phases. A signal analyzer 10 acquires time domain data of the signal under test that has N ideal phases. The signal analyzer 10 demodulates the signal under test to measure the phases θm and amplitudes Am of the symbols of the signal. Phase differences dθ between the ideal and measured phases of the measured symbols are calculated. K group phases θgi are assigned to N of the ideal phases. The signal analyzer displays a constellation using phases θgi+dθ and the measured amplitudes Am. That is, points are plotted at locations of X=Am*cos(θgi+dθ)+jAm*sin(θgi+dθ) on a complex plane.