To provide a construction method for a dam, by which, fertile agricultural land and forest are not submerged, and more cost effective than large scale dams, with functions of electric power adjustment, making use of water, and sand protection, flood controlling, preventing destruction of the eco system and environment, perfectly and permanently preventing sand drifting.

A dam 2 is constructed on the upper-most stream part of the river and the lengthen of a water conduct pipe 5 is extended from the upper- most stream to a hydraulic power generating plant 8 in the plane down from the mountain area. Rocks and sand of the riverbed are crushed into smaller ones sized under 5 cm in diameter, so that they are easily absorbed by the water conduct pipe 5, absorbing and exhausting the drifts of the riverbed 1, making the riverbed 1 inclined adversely. The earth and sand which are absorbed by the water conduct pipe 5 are separated by a drift separater 7, before they are sent to a water turbine of the hydraulic power plant 8. Therefore, the water is conducted to the power plant 8. The rocks ad sands, drifts are sent to surge tanks 13 and 16.