A quantum cascade laser comprising: a plurality of active layers, each of the active layers including a first barrier layer, a first quantum well layer, a second barrier layer, a second quantum well layer, a third barrier layer, a third quantum well layer, and a fourth barrier layer provided in this order along a predetermined direction (z); a plurality of injection layers; and a core layer having the active layers and the injection layers, the active layers and the injection layers being alternately provided on an n-type InP substrate along the predetermined direction to form a cascade structure, wherein the first quantum well layer has a film thickness larger than a film thickness of the second quantum well layer, the second quantum well layer has the film thickness larger than a film thickness of the third quantum well layer, and the second barrier layer has a film thickness smaller than a film thickness of the third barrier layer; wherein the first barrier layer is provided adjacent to the injection layer, the first barrier layer functioning as an injection barrier layer, and the first quantum well layer is provided adjacent to the first barrier layer; electrons of an eigenenergy at an upper energy level injected into the active layer from the injection layer by a resonant tunneling effect with application of an electric field in a direction opposite to the predetermined direction (z) are mainly distributed to a region occupied by the first quantum well layer, the second barrier layer, and the second quantum well layer, luminescence being generated by the electrons transferring from the upper energy level to a lower energy level of the active layer, a wave function of the electrons at the upper energy level and a wave function of the electrons at the lower energy level are formed substantially symmetrical with respect to a reference axis passing through the center in a thickness direction of the second barrier layer; the wave function of electrons at the upper energy level has even parity, and the wave function of electrons at the lower energy level has odd parity; the first to fourth layers (B21, B22, B23, B24) all are aluminum indium arsenide semiconductor layers, AlInAs; the first to third quantum well layers (W21, W22, W23) all are gallium indium arsenide semiconductor layers, GaInAs; the first quantum well layer has a film thickness in the range of not less than 1.05 times and not more than 1.15 times the film thickness of the second quantum well layer; and the second barrier layer has a film thickness in the range of not less than 0.1 nm and not more than 1.2 nm. a plurality of active layers, each of the active layers including a first barrier layer, a first quantum well layer, a second barrier layer, a second quantum well layer, a third barrier layer, a third quantum well layer, and a fourth barrier layer provided in this order along a predetermined direction (z); a plurality of injection layers; and a core layer having the active layers and the injection layers, the active layers and the injection layers being alternately provided on an n-type InP substrate along the predetermined direction to form a cascade structure, wherein the first quantum well layer has a film thickness larger than a film thickness of the second quantum well layer, the second quantum well layer has the film thickness larger than a film thickness of the third quantum well layer, and the second barrier layer has a film thickness smaller than a film thickness of the third barrier layer; wherein the first barrier layer is provided adjacent to the injection layer, the first barrier layer functioning as an injection barrier layer, and the first quantum well layer is provided adjacent to the first barrier layer; electrons of an eigenenergy at an upper energy level injected into the active layer from the injection layer by a resonant tunneling effect with application of an electric field in a direction opposite to the predetermined direction (z) are mainly distributed to a region occupied by the first quantum well layer, the second barrier layer, and the second quantum well layer, luminescence being generated by the electrons transferring from the upper energy level to a lower energy level of the active layer, a wave function of the electrons at the upper energy level and a wave function of the electrons at the lower energy level are formed substantially symmetrical with respect to a reference axis passing through the center in a thickness direction of the second barrier layer; the wave function of electrons at the upper energy level has even parity, and the wave function of electrons at the lower energy level has odd parity; the first to fourth layers (B21, B22, B23, B24) all are aluminum indium arsenide semiconductor layers, AlInAs; the first to third quantum well layers (W21, W22, W23) all are gallium indium arsenide semiconductor layers, GaInAs; the first quantum well layer has a film thickness in the range of not less than 1.05 times and not more than 1.15 times the film thickness of the second quantum well layer; and the second barrier layer has a film thickness in the range of not less than 0.1 nm and not more than 1.2 nm.