According to an embodiment, a semiconductor light emitting device is configured to emit light by energy relaxation of an electron between subbands of a plurality of quantum wells. The device includes an active layer and at least a pair of cladding layers. The active layer is provided in a stripe shape extending in a direction parallel to an emission direction of the light, and includes the plurality of quantum wells; and the active layer emits the light with a wavelength of 10 μm or more. Each of the cladding layers is provided both on and under the active layer respectively and have a lower refractive index than the active layer. At least one portion of the cladding layers contains a material having a different lattice constant from the active layer and has a lower optical absorption at a wavelength of the light than the other portion.