This paper describes the fluctuation characteristics of long-period components of current which are longer than diurnal period. Field measurements of current velocity were carried out at 10 stations for 1 to 2 months in Tokyo Bay. 　It is said that the main causes to create the long-period components of current are density current, whic is caused by salinity and water temperature distributions, wind induced current, tidal residual current, and a branch current of sea current. 　In the paper, principal component analysis is adopted to examine the influence of several kinds of variables such as water temperature, salinity, wind velocity and wind direction, and water surface level difference on long-period components of current. Especially, in Tokyo Bay, the influence of wind effect is predominant to the fluctuation of the long-period current. Therefore the relationships between the fluctuation of averaged current and that of averaged wind are investigated. 　As a result, it is found that the horizontal and vertical circulations in Tokyo Bay are caused mainly by wind cation during the observations. Therefroe, the mechanism of current in the bay deeply depends on the benavior of wind of each season.