A quantum cascade laser is configured with a semiconductor substrate, and an active layer provided on a first surface of the substrate and having a cascade structure in the form of a multistage lamination of unit laminate structures each of which includes an emission layer and an injection layer. The active layer is configured to be capable of generating first pump light of a frequency ω1 and second pump light of a frequency ω2 by intersubband emission transitions of electrons, and to generate output light of a difference frequency ω by difference frequency generation from the first pump light and the second pump light. Grooves respectively formed in a direction intersecting with a resonating direction in a laser cavity structure are provided on a second surface opposite to the first surface of the substrate.