Small Scale Hydroelectric Power Generation is made using the water drop caused by the gradient difference between that of the river and that of the penstock. The water flow in this system apt to vary with time. Especially, at the time of the flood or that of the dry, the flow in the water stream varies largely. To realize the stable generation, it is necessary to maintain the water level in the head tank located in the mid way in the stream channel at a constant level. In this paper, variation of water level is forecasted using two-tank model for the penstock and the head tank and the water level control algorithm is proposed by deadbeat control. The effectiveness of the algorithm is evaluated beforehand using the simulation program of the water environment for the hydroelectric generation system.