Historical 137Cs data obtained in the Japan Sea were statistically analyzed. 137Cs concentrations in the upper 200 m layer decreased almost monotonously after 1970's, while those in the deeper layers below 1500 m depth sharply increased during the period of 1985--1995, accompanied with the increase of the 137Cs inventory. It follows that considerable amount of 137Cs-rich surface water was conveyed into the deep layers after 1985, suggesting that the deep water formation occurred. The spatial distribution of 137Cs in the deep layers in 1990's indicates that the recent deep water formation occurred in the northwestern area of the Japan Sea, around Peter the Great Bay. ¿ 2000 American Geophysical Union