Provided is a battery pack having an air communication passage communicating the interior of the battery pack accommodating battery cells therein with the exterior of the battery pack, and configured to prevent moisture from reaching the battery cells. The battery pack (20) is provided with a housing (100) having an outer bottom plate (145) and an inner bottom plate (141), and defining a primary chamber (97) for accommodating battery cells (98) above the inner bottom plate, and a secondary chamber (148) between the outer bottom plate and the inner bottom plate; wherein the outer bottom plate includes an outer flat plate (146) extending substantially in parallel to the inner bottom plate and an upright wall extending from a peripheral part of the outer flat plate to abut against a peripheral part of the inner bottom plate, the inner bottom plate being provided with at least one opening surrounded by a tubular flange projecting toward the outer bottom plate, the housing including an air communication passage (A) communicating the secondary chamber with an exterior of the housing; and wherein a part of a bottom surface of the inner bottom plate adjacent to the upright wall inclines upward with respect to a reference surface (144) of the bottom surface of the inner bottom plate away from the outer bottom plate toward the upright wall, the air communication passage including a first communication hole (149) having an edge located above the reference surface of the inner bottom plate.