A laser oscillator which can suppress the resonator loss caused by grain boundary scattering while securing a large gain volume by using a granular laser gain medium. A suspension (1) prepared by scattering many dielectric particles doped with a luminous element in a fluid having a refractive index matching that of the dielectric particles is enclosed in a cell (6). Then the cell (6) containing the suspension (1) is set in an optical resonator constituted of a rear mirror (3) and an output mirror (4). When the cell (6) is irradiated with excitation light (2), the dielectric particles in the suspension (1) are laser-excited and output laser light (5). Since the dielectric particles are made to coexist in the index matching fluid, the occurrence of grain boundary scattering is suppressed and a low-loss optical resonator is obtained. It is also possible to increase the diameter of the dielectric particles scattered in the index matching fluid-filled liquid.
