A nitride semiconductor device includes a stem. A heat sink is provided on the stem. At least one nitride semiconductor light-emitting element is connected to the heat sink. A light-detecting element for detecting light from the semiconductor light-emitting element is provided on the stem. A cap for encapsulating therein the heat sink, the semiconductor light-emitting element, and the light-detecting element in a sealed manner is connected to the stem. The space in the cap has an encapsulated atmosphere. The encapsulated atmosphere contains a component for inhibiting diffusion of hydrogen atoms contained in the semiconductor light-emitting element. The present invention suppresses defect due to an increase in operation voltage to increase a ratio of good goods thereby improving the fabrication yield of the semiconductor light-emitting device.