Abstract: A semiconductor laser includes: a stacked body having an active layer including a quantum well layer, the active layer having a cascade structure including a first region capable of emitting infrared laser light with a wavelength of not less than 12 ?m and not more than 18 ?m by an intersubband optical transition of the quantum well layer and a second region capable of relaxing energy of a carrier alternately stacked, the stacked body having a ridge waveguide and being capable of emitting the infrared laser light; and a dielectric layer provided so as to sandwich both sides of at least part of side surfaces of the stacked body, a wavelength at which a transmittance of the dielectric layer decreases to 50% being 16 ?m or more, the dielectric layer having a refractive index lower than refractive indices of all layers constituting the active layer.