A nitride semiconductor laser device includes a nitride semiconductor laser element having a resonator end surface and capable of emitting light with a wavelength of at most 420 nm, a heat sink joined to the nitride semiconductor laser element, a stem with the heat sink mounted thereon, and a light detecting element mounted on the stem for detecting a laser beam from the nitride semiconductor laser element. The nitride semiconductor laser element, the heat sink and the light detecting element are enclosed within a cap that is joined to the stem, and an atmosphere within the cap has a dew point of at most -30.degree. C. and an oxygen concentration of at most 100 ppm.