PURPOSE: To execute the searching work easily, quickly and surely even with greater underground noise, by executing water leakage detection through giving air bubbles to flowing water.

CONSTITUTION: Air bubbles 11 are blown in flowing water 10 in an underground buried water supplying pipe from outside 4. The air bubbles 11 move together flowing water toward the current direction of the flowing water 10 and they are discharged in an underground cavity part 7 with leaked jetting water 6. If no water pool 8 exists in underground cavity 7, the leaked jetting water 6 collides to a surrounding wall 9 to generate the sound of water leakage and defoaming sound of the air bubbles discharged to the part 7 is generated. This is caught with a leakage sound detector 1 placed on the surface 2 of land. If the water pool 8 is moderately large and the water pressure of the flowing water 10 is low, the sound of the water leakage can not be detected, but the air bubbles 11 float and fluctuate in water in the water pool 8 and it collides with the surrounding wall 9, the air bubbles 11 are defoamed, the defoaming sound is generated, and said defoaming can be listened to a water leakage detector 1, then the leaked part 5 can be searched.