PURPOSE: To detect defects of joint parts before water leakage on the basis of intensity of radiant rays and the position of a detector, by inserting a heat source, which uses a radio isotope, and the detector into a pipe which is buried in joint parts of an underground continuous wall.

CONSTITUTION: A pipe with the tip closed consisting of, for example, steel or vinyl chloride is driven into the ground adjacently to joint face 2 between adjacent unit wall materials 1 and 1 before driving concrete into the ground. Heat source 4 using a radio isotope and radiant-ray detector 5 are inserted into one pipe 3 and the other pipe 3' respectively from the ground surface up to the same prescribed depth, and they are pulled up gradually in the same speed while detecting radiant rays, which are radiated from heat source 4, by detector 5. Since radiant-ray detection near a hole, which nay cause water leakage occurrence, is increased rapidly if the hole exists on the wall, the position and dimension of the hole can be estimated by counter unit 7, which is installed on the ground, and the magnitude of the count value respectively.