Abstract: An external resonator type light emitting system includes a light source oscillating a semiconductor laser light by itself and a grating device providing an external resonator with the light source. The system performs oscillation in single mode. The light source includes an active layer oscillating the semiconductor laser light. The grating device includes an optical waveguide having an incident face to which the semiconductor laser is incident and an emitting face of emitting an emitting light of a desired wavelength, a Bragg grating formed in the optical waveguide, and a propagating portion provided between the incident face and the Bragg grating. Formulas (1) to (5) are satisfied.