An illuminating device adapted to a projecting apparatus including a lamp module, a power module, and a fan is provided. The lamp module is separately disposed in the projecting apparatus. The lamp module has a lamp, an igniting module and a plate. The igniting module is capable of generating a high voltage signal to drive the lamp. The plate is disposed between the lamp and the igniting module for shielding radiating heat generated by the lamp. The power module is capable of generating a low voltage signal transmitted to the igniting module through a low voltage power line. The igniting module transforms the low voltage signal into the high voltage signal to drive the lamp. The fan is disposed at a first side of the lamp for cooling the lamp.