A quantum cascade laser comprising: an active layer including a plurality of injection quantum well regions and a plurality of light-emitting quantum well regions, the each of the plurality of injection quantum well regions and the each of the plurality of light-emitting quantum well regions being alternatively stacked, the each of the plurality of light-emitting quantum well regions emitting laser light by optical transition between sub-bands of a carrier, and the each of the plurality of injection quantum well regions mitigating the carrier after the optical transition between sub-bands to a mini-band level, and injecting the carrier into a light-emitting quantum well region downstream, the active layer including a first portion and a second portion, the first portion and the second portion being stacked upon each other, a thickness of the first portion and a thickness of the second portion being same, a refractive index of the first portion and a refractive index of the second portion being substantially same; a first and a second cladding layer (23, 26) provided on both outer sides of the active layer, and having a refractive index lower than an effective refractive index of the each of the plurality of light-emitting quantum well regions; and an optical guide layer interposed between the first portion and the second portion in a stacking direction, having a refractive index higher than the effective refractive index of the each of the plurality of light-emitting quantum well regions, and having a thickness greater than thicknesses of all well layers of quantum well layers of the each of the plurality of light-emitting quantum well regions, the quantum cascade laser emitting light at a lower energy than that of the bandgap of the optical guide layer such that light absorption in the optical guide layer is suppressed. an active layer including a plurality of injection quantum well regions and a plurality of light-emitting quantum well regions, the each of the plurality of injection quantum well regions and the each of the plurality of light-emitting quantum well regions being alternatively stacked, the each of the plurality of light-emitting quantum well regions emitting laser light by optical transition between sub-bands of a carrier, and the each of the plurality of injection quantum well regions mitigating the carrier after the optical transition between sub-bands to a mini-band level, and injecting the carrier into a light-emitting quantum well region downstream, the active layer including a first portion and a second portion, the first portion and the second portion being stacked upon each other, a thickness of the first portion and a thickness of the second portion being same, a refractive index of the first portion and a refractive index of the second portion being substantially same; a first and a second cladding layer (23, 26) provided on both outer sides of the active layer, and having a refractive index lower than an effective refractive index of the each of the plurality of light-emitting quantum well regions; and an optical guide layer interposed between the first portion and the second portion in a stacking direction, having a refractive index higher than the effective refractive index of the each of the plurality of light-emitting quantum well regions, and having a thickness greater than thicknesses of all well layers of quantum well layers of the each of the plurality of light-emitting quantum well regions, the quantum cascade laser emitting light at a lower energy than that of the bandgap of the optical guide layer such that light absorption in the optical guide layer is suppressed.