A structure of an outdoor illuminating device has a lamp housing disposed on a substrate. The light-emitting element of a light source module is disposed under the substrate. The heat sink of the light source module is interposed between the substrate and the lamp housing. A transparent mask covers the substrate from below, and is fixed to the substrate by a fastening element, thereby forming a closed chamber that prevents moisture from entering. The light-emitting element is inside the chamber, and the heat sink is outside the chamber.