PURPOSE: To reduce construction cost by a method wherein pump plants are provided on inlets of an underground drainage canal, and pressure conveyance of water is made toward the underground drainage canal by means of difference between the water level in discharge tanks and the water level in the downstream, or by means of pumping.

CONSTITUTION: An underground canal 4 in a large capacity and vertical shafts 3 for letting foul water fall to the underground canal 4 from a drainage canal in the vicinity of the ground surface are provided to the underground in extreme depth. Incoming foul water, pumped up by pumps in pump plants 1 provided to each of inlets, is allowed to flow into the vertical shafts 3 through open type discharge tanks 2, and is allowed to flow to rivers or the sea through the underground canal 4. Since the pumps are installed to the inlets of the vertical shafts, pumping facilities can be made small without the need for installing equipment for preventing backflow at each of the inlets of the vertical shafts.