At Port Mizushima in Okayama Prefecture as a center of an heavy industrial area in the Seto Inland Sea, many factories have been built and more are in construction for the future progress. This industrial progress has brought water pollution which is one of the public nuisances, a serious problem from the stand point of public health. The anther has investigated the actual feature of the pollution of sea water in this port for six years. The results ware as follows: (1) Water at this port area has been polluted year by year during last six years. (2) The degree of the pollution changes by places: at the innermost the most serious and the entrance the least. (3) At the low-tide water was more polluted than high-water.(4) Industrial wastes were investigated and some of them were out of hygienic criteria.