There is provided an optical microphone for detecting an acoustic wave propagating in an ambient fluid, the optical microphone including: a propagation medium section; a light source for emitting a light wave to be transmitted through a diffraction region in the propagation medium section; and a photoelectric conversion section for detecting the light wave having been transmitted through the propagation medium section. A first acoustic wave which is a portion of the acoustic wave and a second acoustic wave which is another portion thereof are allowed to propagate in the propagation medium section so as to simultaneously arrive at the diffraction region, and an interference component between a +1st order diffracted light wave and a −1st order diffracted light wave of the light wave generated based on a refractive index distribution of the propagation medium occurring in the diffraction region.