[en] For the measurement of the dilution of jet current from a fixed source in the sea, the changes in the shape of plume and the axial concentration were measured in Tokai coastal sea. Sodium fluorescein was used as a tracer. The dilution of rising jet current was represented with Aabraham and Shudo equations. The pattern of diffusion after plume arrived at the sea level is given in figures and a table. Water temperature and salinity in the area near a nozzle are given in a figure. The results of measurement of dye plume diffusion are tabulated. An equation has been proposed for the relationship between the concentration on the stream axis and the distance in the coastal sea on the basis of the dye diffusion experiments made many times in the past and following up of water stream. From the results, the 3/2 power rule may be valid for the marine area under study. According to the foregoing results, the approximation by a two-dimensional diffusion model holds true for the summer season, in which the sea is calm and stratification develops due to insolution, while it may be necessary to use a three-dimensional model for the winter season, in which active vertical mixing occurs. (Kobatake, H.)