PURPOSE: To perform measurement even in a narrow measuring place, by collecting the acoustic data on an imaginary disc, and analyzing the data, without deteriorating resolution and strain characteristics.

CONSTITUTION: A sound source 7 is arranged on an extended part of a measuring disc and an arm 13 is rotated. As a result, when the arm 13 is rotated by one turn, an imaginary disc 9 is drawn. At this time, at every time when a beam switch 18 detects a marker 17c, the rotation of the arm 13 is stopped. Sound data is obtained by microphones 10 and 10a at every time of the stop. In order to rotate the stopped arm 13, a starting switch is operated. The data, which are obtained at a plurality of points on the imaging disc, are analyzed by an analyzer and displayed on a display device.