PURPOSE: To materialize the longitudinal single mode oscillation of a solid laser, the inner resonator type of SHG laser, and the reduction of the laser noise.

CONSTITUTION: In a semiconductor laser excitation inner resonator type of SHG laser, an etalon 122 is used vertically with the advance direction of an oscillated laser, inside resonator, and output light is monitored and it is fed back to the change of the length of the resonator. Accordingly, by shifting the resonator mode, according to the mode selected by the etalon, the solid laser can be oscillated in longitudinal single mode. By the accomplishment of the longitudinal single mode, the of the contention modes disappears, and a solid laser device, which can output SHG stably, can be obtained.