To provide a drainage treatment method capable of carrying out excavation not higher than a groundwater level in an impervious layer and carrying out drainage without giving any influence on the ground.

A steel pipe 21 is driven by boring excavation not higher than a pressurized water line 11 to provide a sump 13 around the upper end of the steel pipe, a casing 23 extended upward by covering the upper end of the steel pipe is installed, a lift pumping route 26 connected to the outside from the upper end 23a of the casing is ensured by placing a lifting pump 25 in the upper end of the steel pipe, a slab concrete 18a approximately coinciding with the upper end is placed on the pressurized water line, and the lifting pump and the lift pumping route are removed to close the upper end of the casing when an influence of the pressurized height is eliminated. For that purpose, pressurized water of a permeable layer located in one layer lower than the surrounding earth retaining is lower than the permeable layer directly under the excavated bottom 12 by the steel pipe. Further, the other sump 14 is provided to tie between both sumps with a pipe line 22 to mainly narrow the sump wherein the steel pipe is driven down to the pumping without making induction of spring from the periphery of the earth retaining more than necessity.