A light emitting device includes a waveguide element including a first mirror that is light transmissive, a second mirror that faces the first mirror, and an optical waveguide layer located between the first mirror and the second mirror, the waveguide element allowing light input to the optical waveguide layer to propagate along a first direction and to be emitted through the first mirror; a first photodetector that is located on a path of light to be input to the optical waveguide layer or on another path branching off from the path and outputs a first signal according to an amount of received light; and a second photodetector that is located on a path of light that has propagated through the optical waveguide layer along the first direction and passed the optical waveguide layer and outputs a second signal according to an amount of received light.