We investigated the characteristics of photoluminescent porous silicon by means of photoacoustic spectroscopy. The efficiency of photoluminescence (PL) from porous silicon varied from sample to sample. From porous silicon with little efficiency in PL, a wide absorption edge was observed in photoacoustic spectra. The energy region of the edge was laid from 1.3 eV to 2.4 eV, where photoluminescence was observed. On the other hand, a photoacoustic spectroscopic signal from porous silicon with high efficiency in PL was found to be constant in this energy region. From these results, we believe that non-radiative relaxation process lowers the PL efficiency of porous silicon.