A quantum cascade laser (1A, 1B) comprising: a semiconductor substrate (10, 50) formed of InP; and an active layer (15, 55) 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 (16, 56) each unit laminate structure including the quantum well emission layer and the injection layer in that sequence, wherein the quantum well emission layer and the injection layer are respectively formed so as to have a quantum well structure including quantum well layers (161,162,163, 164) formed of InGaAs and quantum barrier layers (171, 172, 173, 174) formed of InAlAs, each of the plurality of unit laminate structures (16 56) included in the active layer (15,55) has an emission upper level, an emission lower level, and an injection level as an energy level higher than the emission upper level in its subband level structure, light is generated by intersubband transition of electrons from the emission upper level into the emission lower level in the quantum well emission layer, and electrons after the intersubband transition are injected into the injection level in the unit laminate structure of the subsequent stage via the injection layer, and the quantum well emission layer includes n well layers, n being an integer of 2 or more, and a first well layer closest to the injection layer side of the preceding stage is formed as a well layer for formation of the injection level, and a first barrier layer of the quantum well emission layer is formed as an injection barrier layer, wherein the injection barrier layer is formed so that electrons from the injection layer of a preceding stage are injected into the injection level in the quantum well emission layer due to a resonant tunneling effect via the injection barrier layer, and wherein the first well layer adjacent to the injection barrier layer is formed so that a well width of the first well layer is thinner than well widths of other well layers in the quantum well emission layer, and a ground level corresponds to the injection level when the first well layer exists as a single well layer. a semiconductor substrate (10, 50) formed of InP; and an active layer (15, 55) 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 (16, 56) each unit laminate structure including the quantum well emission layer and the injection layer in that sequence, wherein the quantum well emission layer and the injection layer are respectively formed so as to have a quantum well structure including quantum well layers (161,162,163, 164) formed of InGaAs and quantum barrier layers (171, 172, 173, 174) formed of InAlAs, each of the plurality of unit laminate structures (16 56) included in the active layer (15,55) has an emission upper level, an emission lower level, and an injection level as an energy level higher than the emission upper level in its subband level structure, light is generated by intersubband transition of electrons from the emission upper level into the emission lower level in the quantum well emission layer, and electrons after the intersubband transition are injected into the injection level in the unit laminate structure of the subsequent stage via the injection layer, and the quantum well emission layer includes n well layers, n being an integer of 2 or more, and a first well layer closest to the injection layer side of the preceding stage is formed as a well layer for formation of the injection level, and a first barrier layer of the quantum well emission layer is formed as an injection barrier layer, wherein the injection barrier layer is formed so that electrons from the injection layer of a preceding stage are injected into the injection level in the quantum well emission layer due to a resonant tunneling effect via the injection barrier layer, and wherein the first well layer adjacent to the injection barrier layer is formed so that a well width of the first well layer is thinner than well widths of other well layers in the quantum well emission layer, and a ground level corresponds to the injection level when the first well layer exists as a single well layer.