Port Mizushima has been artificially modified during the development of industries. The shape is long and narrow like a canal where no stream ispouring into. The pollution of sea water of the port was investigated for six years and the results were reported in Peport (1). In this report, the auther has devised a method to evaluate the degree of the sea pollution using a system of triangle co-ordinates. The first axis (X) shows places, the second (Y) years and the third (Z) degrees of pollution expressed by disorbed oxygen (DO), chemical oxygen demand (COD). biochemical oxygen demand (BOD) and iodine consumptions. This device has been proved possible to evaluate the degree of pollution simultaneously concerning places, laps of years. He has shown the changing patterns of the sea pollution at Port Mizushima as a whole using this triangle co-ordinates.