A commercial or industrial large city often develops at the hinterland of a deep bay owing to its high defensibility against stormy sea. However, the exchange rate of sea-water between the inside and the outside of the bay is very low, so that the water is much polluted by industrial and domestic waste water in many cases. The Dokai Bay has also the same social problem. In order to solve the problem we propose an idea of improvement of water quality in this paper, which is that clean sea water of Hibiki open sea is introduced into Dokai Bay. Various conditions for water purifications have been attempted by using the diffusion simulations of COD. It has been clear that introduction of clean sea water (3m3/sec.) into the innerpart of Dokai Bay is very effective for purification.