PURPOSE: To improve fault analytical capacity by detecting a signal accompanied by a light absorbing current generating in a sample device through the irradiation of a laser beam indicating a light absorbing current image based on a signal accompanied by the detected light absorbing current.

CONSTITUTION: A light absorbing current image is observed on an oscilloscope 36 under varying action conditions on a sample device 11 by changing input conditions against the sample device 11 in an input signal generation circuit 41. For this reason, it is possible to measure the electrical characteristics of the interior of the sample device 11 and further detect a fault under a static action state in respective input conditions by detecting a difference in the modulation degree of a light absorbing current. A sound image is indicated on the oscilloscope 24 and therefore, it is possible to observe a sound image and at the same time, observe a light absorbing current image. Thus the physical defect of the sample device 11 can be detected as well as the malfunction or fault of electrical characteristics.