An illuminance device including a gas discharge lamp, an airflow generator, and a distributing duct is provided. The lamp has a reflector, a base connected to the reflector, a burner installed in the reflector, and the base, a first opening located at the reflector and a front part of the lamp, and a second opening located at a rear part of the lamp and exposing the burner. The distributing duct has an inlet, a first outlet, and a second outlet, and an opening area of the first outlet is larger than that of the second outlet. A cooling airflow provided by the airflow generator enters the distributing duct via the inlet, the distributing duct guides a part of the cooling airflow to the first opening via the first outlet, and the distributing duct guides another part of the cooling airflow to the second opening via the second outlet.