A quantum cascade laser (1A, 1B) comprising: a semiconductor substrate; and an active layer which is provided on the semiconductor substrate and has 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 included in the active layer has, in its subband level structure, a first emission upper level (L up1), a second emission upper level (L up2) as an energy level higher than the first emission upper level (L up1), a single emission lower level (L low), and a relaxation level (L r) as an energy level lower than the emission lower level (L low), each of the quantum well emission layer and the injection layer is formed to have a quantum well structure including quantum well layers and quantum barrier layers, the first emission upper level (L up1), the second emission upper level (L up2), and the emission lower level (L low) are energy levels in the quantum well emission layer, light is generated by intersubband transitions of electrons from the first emission upper level (L up1) and the second emission upper level (L up2) to the emission lower level (L low) in the quantum well emission layer, and electrons after the intersubband transitions are relaxed from the emission lower level (L low) to the relaxation level (L r), and injected from the injection layer into the quantum well emission layer of the unit laminate structure of the subsequent stage via the relaxation level (L r), and wherein the unit laminate structure included in the active layer has, in its subband level structure, a plurality of emission upper levels including at least the first emission upper level (L up1) and the second emission upper level (L up2), and concerning emission intensities of intersubband transitions from the plurality of emission upper levels (L up1, L up2) to the emission lower level (L low), the weakest emission has an emission intensity not less than 1/5 of that of the most intense emission, wherein electrons are injected into the first emission upper level (L up1) and the second emission upper level (L up2) and wherein the first emission upper level (L up1) and the second emission upper level (L up2) behave like one emission upper level with an energy width, so that an emission spectrum becomes a spectrum homogeneously spread. a semiconductor substrate; and an active layer which is provided on the semiconductor substrate and has 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 included in the active layer has, in its subband level structure, a first emission upper level (L up1), a second emission upper level (L up2) as an energy level higher than the first emission upper level (L up1), a single emission lower level (L low), and a relaxation level (L r) as an energy level lower than the emission lower level (L low), each of the quantum well emission layer and the injection layer is formed to have a quantum well structure including quantum well layers and quantum barrier layers, the first emission upper level (L up1), the second emission upper level (L up2), and the emission lower level (L low) are energy levels in the quantum well emission layer, light is generated by intersubband transitions of electrons from the first emission upper level (L up1) and the second emission upper level (L up2) to the emission lower level (L low) in the quantum well emission layer, and electrons after the intersubband transitions are relaxed from the emission lower level (L low) to the relaxation level (L r), and injected from the injection layer into the quantum well emission layer of the unit laminate structure of the subsequent stage via the relaxation level (L r), and wherein the unit laminate structure included in the active layer has, in its subband level structure, a plurality of emission upper levels including at least the first emission upper level (L up1) and the second emission upper level (L up2), and concerning emission intensities of intersubband transitions from the plurality of emission upper levels (L up1, L up2) to the emission lower level (L low), the weakest emission has an emission intensity not less than 1/5 of that of the most intense emission, wherein electrons are injected into the first emission upper level (L up1) and the second emission upper level (L up2) and wherein the first emission upper level (L up1) and the second emission upper level (L up2) behave like one emission upper level with an energy width, so that an emission spectrum becomes a spectrum homogeneously spread.