PURPOSE: To reduce the construction cost, by a method wherein a strainer casing, having water pass holes, formed in a position located facing a water-bearing stratum, and the interior, partitioned by a partitioning material into an upper and a lower part, is inserted into a vertical shaft, and underground water pumped up from the upper water-bearing stratum is returned to the lower water-permeable stratum.

CONSTITUTION: A strainer casing 7, having water pass holes 3, formed in a position facing upper and lower water-bearing strata A and B and a partioning member 4, situated in a position located facing a non-water permeable stratum (a) and partitioning the interior into an upper and a lower part, is inserted into a vertical shaft 2. A vertical pipe 5, to which an air feed pipe 6 is connected, and an air bleed pipe 15 are installed to the partitioning member 4 in a manner to extend therethrough. A space between the casing 7 and a hole wall is filled with a filter member 10, and a packer 11 is situated in a position located facing the non-water permeable stratum (a). Water is drained from the water-bearing stratum A with the aid of a pump 12 to lower a water level Pa, and after sand in the drained underground water is settled in a water tank 9 for separation, the underground water is returned to the water-bearing stratum B through the vertical pipe 5. When a casing 7b is washed, through the feed of the air in the vertical pipe 5, underground water in the eater-bearing stratum B is caused to flow in the casing 7b by means of air lift.