The shear layer of two-dimensional air jet impinging on a plate with a rectangular hole under the condition of a self-sustained oscillation is subjected to pure-tone acoustic excitation at the exit, and the unsteady jet response is characterized in terms of the excitation frequency and the excitation voltage. A locked-in response at excitation frequency (fe) or 1/n (n=1∿12) oscillation of a self-sustained frequency (fo) appears. A sound pressure level (SPL) has the minimum at fe/fo=0.54 and 0.9 in 2 Volts of excitation voltage, and in those cases the component fo of SPL spectrum decreases.