In order to make clear the physical mechanism of aoshio in Lake Abashiri, the present authors carried out field observations in which flow characteristics and the behaviors of density interface were focused on. We found that the density interface oscillates in the first mode of internal seiche, and wind-driven current that form two vertical circulation occurs, when strong wind is blowing on the lake for long time. Furthermore, the influences of the topography of the lake, the inflow discharge from the upstream rivers, and internal seishe on wind driven current in this lake are made clear.