A nitride semiconductor device includes a GaN substrate (11) in which an angle between a principal surface and an m-plane of GaN is -5° or more and +5° or less, a first intermediate layer (101) disposed on the principal surface of the substrate (11) and made of Al z Ga (1-z) N (0 ≤ z ≤ 1), and a second intermediate layer (102) disposed on a principal surface of the first intermediate layer (101), having an Al content different from that of the first intermediate layer (101), and made of Al x1 In y1 Ga (1-x1-y1) (0 ≤ x1 ≤ 1, 0 ≤ y1 ≤ 1). A quantum cascade laser (10) includes the nitride semiconductor device.