To suppress noise contained in wave motion for interference to be a minimum extent and to make the detection performance of an interferometer improved.

This vibration detecting device is constituted by constituting two interferometers constituted of an optical path provided with a vibrating mirror 5 and an optical path without a vibrating mirror 5 with respect to the light radiated from a laser beam source 1 to detect vibration of the vibration mirrors 5, by detecting the interfering light having each different phase by two interferometers. This vibration detecting device is provided with photodetectors 13, 14, 15, 16 for detecting the light in two paths, tracking the same reflection path as two optical paths constituting each interferometer and forming no interference concerning the light radiated from the laser beam source 1 and difference output light detecting units 19, 20 for subtracting the detection signals of the light in two paths detected by the photodetectors 13, 14, 15, 16 and forming no interference from the detection signals of the interfering light detected by each interferometer.