PURPOSE: To obtain the transfer function of a system readily, by performing square-law detection of the sum of an input signal and a signal, which is obtained by delaying said input signal, or another signal, taking out only a low-frequency signal, performing AD conversion, and performing Fourier transformation.

CONSTITUTION: The output of a buffer 3 is guided to a delay element 5 with a switch 13. The output of a buffer 4 is selected with a switch 14. A square-law detector 6 performs square-law detection of the sum of an input signal A, which is delayed through the delaying element 5, and an input signal B. The result is made to pass through a low-pass filter 7 and made to be a digital signal through an AD converter 8. The result is stored in a memory 9. When the data measurement is finished, high speed Fourier transformation is performed in a signal processor 10. The mutual power spectrums of the signal A and the signal B are obtained by the Wiener-Khintchine's theorem and displayed on a display 11. When both switches 13 and 14 select the input signal A, a self-power spectrum is obtained by the same way.