Many studies have ever been performed in the Ariake Sea where deterioration of aqueous environment is conspicuous, but properties of tidal current have not been made clear yet. In this study, we observed the current velocity at more than one spot in a conspicuous Isahaya Bay of water quality deterioration at summer and winter of 2013 for the purpose of explication of properties of tidal current. First, a harmonic analysis of the current velocity data was carried out and a tidal current ellipse was drawn to know properties of tidal current at Isahaya Bay. Vertical structures of tidal ellipses showed that the current direction difference is large in summer, and small in winter. Second, we examined the factors that affected residual current using multiple regression analysis. The multiple regression analysis with tidal amplitude, wind velocity, river discharge and drainage discharge as an explanatory variable and with residual current as a target variable was performed. As a result of the multiple regression analysis, it is clarified that most of the residual current of the surface is explained by a wind.