PURPOSE: To enable the exact selective measurement of a test body by determining the imaginary part of the cross spectrum of respective detection signals in accordance with the detection signals having a time difference by a pair of noise detecting means which are spaced at a prescribed distance on the perpendicular of the surface of the test body.

CONSTITUTION: The signals inputted via noise detecting microphones 2a, 2b installed apart from each other at a space Δl on the perpendicular from the surface of a test body 1 and amplifiers 3a, 3b amplifying the outputs thereof are fed via an A/D converter 6 to a Fourier transform device 7 and thereafter the signals are fed to a cross spectrum imaginary number converter 8 which determines the difference in a pair of the converted outputs. The influence of the dark noise by other kinds of surrounding apparatus is eliminated by the outputs of the converter 8 and the noise from the body 1 is selectively measured. Since the imaginary part of the cross spectra in the detection signals in determined in the above-mentioned way, the noise of the test body is exactly and selectively measured irrespectively of the presence or absence of the dark noise.