PURPOSE: To reduce a volume of an underground structure at deep level and a power cost on a drainage pump, and to effectively utilize stored water.

CONSTITUTION: An underground impounding reservoir 10 is provided to the middle position of a flow-in main pipe passage 1 and the ground level in communicated to the flow-in main pipe passage 1. A dam and the like is provided to the bottom part of the underground impounding reservoir 10, a fixed quantity of stored water is made possible to be held, and utilized effectively. Rated total pump head of a drainage pump 8 is selected in matched to a standard water level HWL1 set to an underground impounding reservoir 10 level and operation of the drainage pump 8 is started in accordance with this. Therefore, reduction in drainage pump installation cost and drainage power cost, reduction in excavating work amount and so on in great depth and construction cost, improvement in workability, and shortenizing in construction term become possible.