Disclosed is a battery pack which comprises a plurality of secondary batteries, a case for housing the plurality of secondary batteries, and at least one heat insulation layer for separating the plurality of secondary batteries from each other. The at least one heat insulation layer contains a foam material which is expandable at a first predetermined temperature of 110 DEG C or greater but less than 200 DEG C, and air cells are generated within the heat insulating layer when the foam material is expanded, thereby increasing the thickness of the heat insulating layer. Consequently, the battery pack exhibits a high heat insulation effect at the time of abnormal heat generation in a battery, and thus the battery pack can be excellently safe.