A nitride semiconductor device according to the present invention includes a n-GaN substrate 10 and a semiconductor multilayer structure arranged on the principal surface of the n-GaN substrate 10 and including a p-type region, an n-type region and an active layer between them. The laser and the mirror are arranged such that a laser beam emitted from the laser may be reflected by the mirror. The second chamber is connected to the hot chamber by two conduits, such that a fluid filling the two chambers can circulate between the two chambers. This invention enhances cold start performance and operation with a pre-start thermal conditioner with warming capabilities.