PURPOSE: To detect a flaw in the whole area including the direction of the thickness of a specimen and also to enable measurement of a depth position of the detected flaw by a method wherein a reflection echo of a transmitted wave transmitted twice through the specimen through the intermediary of a reflecting plate and a flaw echo are taken out separately from each other.

CONSTITUTION: A probe 8 is disposed on one side of a specimen 1 with a water column 6 interposed between, while a reflecting plate 9 reflecting an ultrasonic wave emitted from the probe 8 and transmitted through the specimen 1 is provided on the opposite side of the substance with a water column 11 interposed between. A transmitted wave reflected from the reflecting plate 9 and transmitted again through the specimen 1 and a reflected wave not transmitted through the specimen 1 but reflected therefrom are received by the probe 8. Out of the reflected waves received, subsequently, a transmitted reflection echo of the transmitted wave transmitted twice through the specimen 1 through the intermediary of the reflecting plate 9 and a flaw echo of a reflected wave reflected from a flaw in the specimen 1 are taken separately from each other. By using the transmitted reflection echo and the flaw echo thus taken out, the flaw and the depth position of the flaw can be detected simultaneously.