A method for detecting a photoacoustic signal includes the steps of modulating the intensity of light obtained from a light source at a predetermined modulation frequency, exciting a specimen by directing the intensity-modulated light onto the specimen, thereby generating a photoacoustic effect in the specimen, detecting the photoacoustic effect generated in the specimen and producing a detection signal indicative of the detected photoacoustic effect, and extracting information about the surface and interior of the specimen from the detection signal, wherein the intensity-modulated light is emitted from an aperture of a near-field optical scanning microscope used in directing the intensity-modulated light onto the specimen while a distance between the aperture of the near-field optical scanning microscope and the surface of the specimen is maintained constant irrespective of the photoacoustic effect generated in the specimen.
