To provide a groundwater-level decreasing method which enables a riser pipe to be efficiently driven into ground below a depth of about 20 m, and which enables a groundwater level to be decreased below the deep ground, for example, with the depth of about 20 m.

This groundwater-level decreasing method comprises: a groundwater-discharge-hole forming step of forming a groundwater discharge hole in the ground of an area, in which the groundwater level is desired to decrease, by boring; a riser-pipe driving step of driving the riser pipe, having a well point attached to its leading end, into the bottom of the groundwater discharge hole formed in the groundwater-discharge-hole forming step, while driving water pumped by a drain pipe inserted into the riser pipe is discharged to the outside from the well point at the leading end; and a drainage step of discharging the groundwater, intruded into the riser pipe, by using the synthetic-resin drain pipe inserted into the drain pipe or the riser pipe after the riser-pipe driving step.