PROBLEM TO BE SOLVED: To analyze the separation degree of an FM stereo signal through the use of a unit constituting a mixed signal IC tester as a standard by receiving the IF signal output of a device to be tested, demodulating arithmetically the FM stereo signal and obtaining the separation degree. SOLUTION: A signal generator 10 generate the FM-modulated stereo signal by using an arbitrary waveform generator, for example. A DUT 90 is the device to be tested. Relating to a digitizer 20, a main part is constituted of an A/D converter and a memory and the analog signal of several ten MHz is A/D- converted from DC. When a signal to be measured is from several hundred MHz to several GHz , a sampling digitizer where a sampling head is added to the digitizer is used. A DSP(digital signal processor) 30 is a private processor executing an algebraic operation processing on the inputted digital signal on a real time basis and at high speed. A display means 40 is a monitor for displaying an analyzed result, or the like.