PURPOSE: To exactly lower the level of ground water by attaching an air leak preventive elastomer to the mouth of a perforated pipe wound with a water- permeable cloth.

CONSTITUTION: A perforated pipe 2 wound with a water-permeable cloth 4 is penetrated into a nearly horizontal hole (not illustrated) previously drilled in a sloped ground by a casing 1, an air lead preventive elastomer 3 is inserted into the cloth 4 folded mouth of the pipe 2, and a coupling 5 attached to the casing 1 is pressingly contacted with the elastomer 3 while turning. Then, a vacuum pump (not illustrated) having an air-water separator is connected through a pipe 7 to the coupling 5 and ground water is forcibly drained by the vacuum pump.