A quantum cascade laser comprising: a semiconductor substrate, and an active layer provided on the semiconductor substrate and having a cascade structure formed by alternately laminating quantum well emission layers and injection layers by multistage-laminating unit laminate structures each including the quantum well emission layer and the injection layer, wherein the unit laminate structure has, in its subband level structure, a first emission upper level (L up1), a second emission upper level (L up2) of an energy higher than the first emission upper level, and a plurality of emission lower levels (L low) of energies lower than the first emission upper level, a first barrier layer in the quantum well emission layer is an injection barrier layer being a barrier layer thickest in the unit laminate structure, light is generated by intersubband transitions of electrons from the first emission upper level and the second emission upper level to the plurality of emission lower levels in the quantum well emission layer, and electrons after the intersubband transitions are injected into the quantum well emission layer of the unit laminate structure of the subsequent stage via the level in the injection layer and via the injection barrier layer due to the resonant tunneling effect, electrons from the level in the injection layer of the preceding stage to the quantum well emission layer are injected into the second emission upper level, the quantum well emission layer includes n well layers, n being an integer of 2 or more, one of the first emission upper level (L up1) and the second emission upper level (L up2) is a level arising from a ground level in a first well layer closest to the injection layer side of the preceding stage, and the other is a level arising from an excitation level in a well layer (162, 163, 164) except for the first well layer, an energy interval ΔE 43 between the first emission upper level and the second emission upper level is set to be smaller than the energy of a longitudinal optical phonon, and for a higher energy level on the higher energy side adjacent to the second emission upper level, an energy interval ΔE 54 between the second emission upper level and the higher energy level (L h) is set to be larger than the energy of a longitudinal optical phonon, and all energy intervals ΔE 2 between levels adjacent to each other in the plurality of emission lower levels are set to be smaller than the energy of a longitudinal optical phonon. a semiconductor substrate, and an active layer provided on the semiconductor substrate and having a cascade structure formed by alternately laminating quantum well emission layers and injection layers by multistage-laminating unit laminate structures each including the quantum well emission layer and the injection layer, wherein the unit laminate structure has, in its subband level structure, a first emission upper level (L up1), a second emission upper level (L up2) of an energy higher than the first emission upper level, and a plurality of emission lower levels (L low) of energies lower than the first emission upper level, a first barrier layer in the quantum well emission layer is an injection barrier layer being a barrier layer thickest in the unit laminate structure, light is generated by intersubband transitions of electrons from the first emission upper level and the second emission upper level to the plurality of emission lower levels in the quantum well emission layer, and electrons after the intersubband transitions are injected into the quantum well emission layer of the unit laminate structure of the subsequent stage via the level in the injection layer and via the injection barrier layer due to the resonant tunneling effect, electrons from the level in the injection layer of the preceding stage to the quantum well emission layer are injected into the second emission upper level, the quantum well emission layer includes n well layers, n being an integer of 2 or more, one of the first emission upper level (L up1) and the second emission upper level (L up2) is a level arising from a ground level in a first well layer closest to the injection layer side of the preceding stage, and the other is a level arising from an excitation level in a well layer (162, 163, 164) except for the first well layer, an energy interval ΔE 43 between the first emission upper level and the second emission upper level is set to be smaller than the energy of a longitudinal optical phonon, and for a higher energy level on the higher energy side adjacent to the second emission upper level, an energy interval ΔE 54 between the second emission upper level and the higher energy level (L h) is set to be larger than the energy of a longitudinal optical phonon, and all energy intervals ΔE 2 between levels adjacent to each other in the plurality of emission lower levels are set to be smaller than the energy of a longitudinal optical phonon.