To provide a vibration detecting apparatus capable of improving detection sensitivity when optical detection of digital vibrations is performed.

A laser beam Lout from a laser light source 10 is made to separately travel through two optical paths OP1 and OP2 in a two-beam interferometer. A diffracted light Ld1 which has traveled through the optical path OP1 and been diffracted by a diffraction element 151 on a vibrating membrane 14 and a diffracted light Ld2 which has traveled through the optical path OP2 and been diffracted by the diffraction element 151 are made to interfere with each other to form interference fringes. On the basis of the interference fringes, vibrations of the vibrating membrane 14 are quantized and detected. Vibrations of the vibrating membrane 14 are thereby optically and digitally detected. The diffraction pitch of the diffraction element 151 is set in such a way as to have approximately the same length as that of the pitch of the interference fringes F1 when the vibrating membrane 14 is in a stationary state. Since the directions of diffraction of the diffracted lights Ld1 and Ld2 approximately match each other, displacements of the vibrating membrane 14 are amplified and detected.