To provide a vibration detector capable of improving detection sensitivity when optically detecting vibration.

A laser beam Lout from a light source 10 is separated into two optical paths (a reference optical path and a reflecting optical path) within an interferometer. Reference light reflected by a reflecting plate 141 in the reference optical path, and reflected light reflected by a diaphragm 131 and a half mirror 142 in the reflecting optical path, interfere with each other to form interference fringe, and vibration of the diaphragm 131 is detected based on the interference fringe. The vibration of the diaphragm 131 is thereby detected optically. Further, the reflected light is to be light multiply reflected between the diaphragm 131 and the half mirror 142 in the reflecting optical path. The optical path difference between the reference light and reflected light is thereby enlarged, and the displacement of the diaphragm 131 is amplified and detected.