An elastic-wave monitoring device includes an optical system which is arranged so that a circularly polarized light from a light source is incident to a measured object and a light beam from the measured object passes through a polarizing filter to a photodetector. A detection unit detects periodic fluctuation components of an output signal of the photodetector. In the elastic-wave monitoring device, the polarizing filter is arranged to have a polarization transmission axis directed to one of directions of principal axes of an ellipse formed by intersections of an index ellipsoid of the measured object and a plane perpendicular to an incidence direction of the light beam and passing through an origin of the index ellipsoid.