The reflection of light from the surface of an elastic solid gives rise to various types of elastic waves that propagate inside the solid. The weakest waves are generally those that are generated by the radiation pressure acting during the reflection of the light. Here, we present the first quantitative measurement of such light-pressure-induced elastic waves inside an ultrahigh-reflectivity mirror. Amplitudes of a few picometers were observed at the rear side of the mirror with a displacement-measuring conical piezoelectric sensor when laser pulses with a fluence of 1 J/cm^{2} were reflected from the front side of the mirror.