PURPOSE: To lower the ground water level, by fitting rod core tube couplings having screwed parts at two stages and connecting a pipe with holes for discharging water and an outer casing to each other by respective prologues to fix them at the rod core tube couplings.

CONSTITUTION: An outer casing 4 is installed at the horizontal water discharging position by boring to excavate a drainage way. Next, a pipe 5 with holes for discharging water is extended to reach the bottom of hole. And previously prepared two prologues 11 are connected to respective the outer casing 4 put at its opening in order and the pipe 5 having holes for discharging water in order to connect the two to a rod core tube coupling 2. And at first, the pipe 5 having holes for discharging water is fitted at the smaller screw cut to a little longer extent in the rod core tube coupling 2 and the casing 5 is also fitted at the larger screw of the rod core tube coupling thereafter to fix them. Moreover, they are connected to a vacuum drain pump to lower the ground water level.