In this paper, the oscillation of an impinging jet on a spherical object is discussed. An underexpanded jet issuing from a convergent nozzle collides against a hemisphere which is glued to a flat plate. The flow field is visualized by the shadowgraph method and the numerical simulation is carried out using TVD scheme, and the frequency of the pressure change at the top of spherical object obtained by the computation is analyzed. As a result, it is found that the oscillation mode of the jet mainly depends on the position of the plate shock, and the feedback mechanism of the density disturbances in the flow field is shown by the computational results.