The photoacoustic signal from the depletion layer beneath the metal electrode in a metal/semiconductor (M/S) structure was detected using the photoacoustic method. To measure the reverse-bias voltage dependence of distribution of the photoacoustic signal from the depletion layer, the surface of the electrode was illuminated and scanned by an intensity-modulated optical-beam. It was obtained that the photoacoustic signal phase differences between depletion layer generating and non-depletion layer were dependent on the depletion layer spreading. One dimensional scanning on the electrode has also revealed that spreading of a depletion layer extends from the each edges of an electrode.