Polders have no basin of their own, and the supply of irrigation water in polders is always short and unstable. The irrigation water system is so structured that the conflicting objects of both the insurance of irrigation water and drainage of rainwater can be achieved. Quoting an example at the surrounding area of the Kojima Bay in the southern area of Okayaja Prefecture where inning (land reclamation by drainage) has been practiced for a long time, problems in drainage improvement required for better use of low-lying flat lands are discussed. There are complicate relations among the safety against flood, rainwater drainage capability, stock capacity, and critical leveling time, therefore, the basic operation which must be performed at first in making a drainage plan is to determine the basic framework of the plan taking those relations into consideration. In low-lying flat lands, safety of the total area against flood has been established basing on the stock capacity of the paddy fields, and it is important to secure paddy field areas to a certain extent. 8 refs., 7 figs., 1 tab.