A quantum cascade laser is configured to include a semiconductor substrate and an active layer which is provided on the substrate and has a cascade structure formed by multistage-laminating unit laminate structures 16 each including an emission layer 17 and an injection layer 18. The unit laminate structure 16 has, in its subband level structure, a first emission upper level Lup1, a second emission upper level Lup2 of an energy higher than the first emission upper level, an emission lower level Llow, and a relaxation level Lr of an energy lower than the emission lower level, light is generated by intersubband transitions of electrons from the first and second upper levels to the lower level, and electrons after the intersubband transitions are relaxed from the lower level to the relaxation level and injected from the injection layer 18 into an emission layer 17b of a subsequent stage via the relaxation level. Accordingly, a quantum cascade laser capable of preferably obtaining emission in a broad wavelength range is realized.