PURPOSE: To achieve an easy to read display of a phase difference. frequency ratio and a phase lead and lag of a cyclic signal with respect to a reference signal.

CONSTITUTION: Reference signals A1 and B1 shifted by 90° in phase are inputted, values of A1 and B1 as obtained the moment a signal C1 to be tested exceeds 0V are sampled and multiplied by a signal of an oscillator 3 for scanning to display a light beam with an angle corresponding to a phase difference on a screen 6 of an X-Y display device 5. The reference signals shifted in phase are inputted to perform a rotary scanning on the screen 6 of the X-Y display device 5 and the signals obtained multiplied by the signal to be tested. Thus, the figure of a petal corresponding to a frequency ratio is displayed with a light spot being deflected vertical to a circumference.