Recently, new type of equipments using density current are applied to improve water quality in enclosed bays such as Gokasho Bay and Ohmura Bay. In order to obtain a knowledge on diffusion by density current in the topography of real sea, hydraulic model experiments were carried out by using the distorted models installed in the rotating tank which could simulate the effect of the earth's rotation. In the experiments, density current was produced which is intruded between the stratified two water layers with different density, for the purpose of examine the effects of the equipments of improving water quality. A new similarity law was proposed to reproduce density current under the distorted model. In the analysis procedure, a technique was established which measured the distribution of dyestuffs concentration by means of the luminance method. Then, effects of tide, density current, the earth's rotation on the material diffusion under the topography of real sea was examined.