PURPOSE: To attain to enhance the efficiency of measuring work by shortening a frequency measuring time of a receiving signal, by calculating the frequency value of an offset signal inputted to a signal synthesizer part by using a frequency synthesizer.

CONSTITUTION: A signal (a) with unknown frequency f1 received by an antenna 1 is applied to a signal synthesizer part 2 and synthesized with an offset signal (b) with frequency f2 from a frequency synthesizer 11 and both signals are inputted to a receiving part as a beat wave form synthetic associated wave (c). The output of a detector 5b is applied to the vertical axis input of a CRT part 6 as a signal (d) with difference frequency f3 of f1 and f2. A reference signal (e) with frequency f4 from a reference frequency generator 7 is inputted to the external sweep input terminal of the CRT part 6. As a result, a Lissajous' figure is displayed on CRT.