PRESSURE RELEASE STRUCTURE FOR VEHICLE BATTERY PACK

To discharge internal gases to the outside through a separable opening of a weakly joined part, when the internal pressure of a case chamber has risen due to production of gases. In a vehicle installed battery pack in which a battery pack module (2) is arranged in a battery pack lower frame (11), a battery pack upper cover (12) is securely joined to the battery pack lower frame (11) via a strong adhesive seal member (81) and a weak adhesive seal member (82) extending continuously about the entire perimeter of respective outside peripheral edge portions (11a), (12a). The battery pack upper cover (12) is provided with a deforming stepped portion (83) having a height difference at least in the heightwise direction, and a weakly joined portion (84) of lower joint strength than other joined portions is set in an area of the strong adhesive seal member (81) and the weak adhesive seal member (82) extending continuously about the entire perimeter, the area corresponding to the deforming stepped portion (83).