  We report a negative resistance, namely, a voltage drop along the opposite
direction of a current flow, in the superconducting gap of NbSe$_2$ thin films
under the irradiation of surface acoustic waves (SAWs). The amplitude of the
negative resistance becomes larger by increasing the SAW power and decreasing
temperature. As one possible scenario, we propose that soliton-antisoliton
pairs in the charge density wave of NbSe$_2$ modulated by the SAW serve as a
time-dependent capacitance in the superconducting state, leading to the dc
negative resistance. The present experimental result would provide a previously
unexplored way to examine nonequilibrium manipulation of the superconductivity.
