A quantum cascade laser of an embodiment includes a semiconductor stacked body in which a ridge waveguide is provided. The semiconductor stacked body includes an active layer including a quantum well region including a layer including Al; and the active layer emits laser light. The layer that includes Al includes first regions, and a second region interposed between the first regions; the first region includes Al oxide and reaches a prescribed depth inward from an outer edge of the active layer along a direction parallel to a surface of the active layer in a cross section orthogonal to the optical axis; and the second region does not include Al oxide.