A self-induced oscillation of a free-surface caused by plunging jet was discovered. Plane water jet flowed into a thin rectangular tank horizontally and fell to the water in the tank. The jet plunged into the water and flowed out through the outlet at the bottom. At a certain water level and flow rate, the free surface began to oscillate and the sloshing motion of water in the tank was excited. The occurrence of the oscillation was dependent on the jet velocity at the plunging point. The oscillating frequency almost agreed with the natural frequency of the liquid in the tank. Oscillation mode shift was observed with varying the tank width. The mode shift occurred so that the sloshing frequency fell in a certain range. Under oscillating conditions, Strouhal number, which represents the ratio of the travelng time of jet disturbance from the plunging point to the outlet to sloshing period, was in the range of 0.2 to 0.5.