A new type of self-induced free surface oscillation (swell flapping) was discovered in a system with a jet, a free surface and a structure. An upward round jet was injected into a cylindrical tank from the bottom center. The jet impinged on the bottom of a cylindrical rod (UIS) which was set just beneath the free surface, resulting in separation of the jet. In a certain condition, the separated jet oscillated, forming a flapping swell of the free surface around UIS, i.e., swell flapping. The inlet jet did not oscillate. The onset condition of the swell flapping depended mainly on the jet inlet velocity and UIS depth. The frequency of the swell flapping was different from that of sloshing, and depended mainly on jet inlet velocity and UIS depth. The oscillation was considered to be excited by the interaction between the separated jet and the free surface.