A method and an apparatus for detecting a photoacoustic signal are provided which irradiate an excitation light beam, modulated by a desired frequency, simultaneously to a plurality of points being measured on a surface of a sample, irradiate the excitation light and a probe light simultaneously to the plurality of the points being measured, detect an interference light of a reflected light beam of the probe light and a specified reference light with a detector made up of a plurality of photoelectric converting elements corresponding to the respective points being measured, the detector being in conjugate relation with the surface of the sample, detect a thermal distortion of the frequency component equal to the intensity-modulated frequency at the plurality of the points being measured from the interference light intensity signal detected by the detector, and detect information relative to the surface and the subsurface of the measuring points on the sample from the thermal distortion of the frequency component. A plurality of measuring points on a specimen are excited simultaneously with an intensity-modulated flat light beam and the thermal expansion displacements of the measuring points are detected simultaneously by interference between a flat probe light beam and a flat reference light beam to detect photothermal displacement signals representing the photothermal displacements of the plurality of measuring points simultaneously.