Light amplification in the ultraviolet region is confirmed in mixed rubidium-cesium chloride crystals at room temperature. The probe laser light, which falls on the 275-nm band of Auger-free luminescence (AFL) arising from radiative transition of the Cl 3p valence electrons into the Cs 5p core holes, is enhanced in intensity when the deep-lying Rb 4p core electrons are pumped into the conduction band by undulator radiation from an electron storage ring. The obtained enhancement factor roughly corresponds to an amplification coefficient of 7×103 cm-1, which is much higher than those of typical solid-state lasers. It is emphasized that the amplification of AFL occurs in a surface layer as thin as about 20 nm, and that the inverted population between the valence and core bands is realized with any pump power. The present observation provides us with a new possibility of nanolaser fabrication.