An object is to, in a battery pack device in which battery cells are lined up, provide a thermally conductive member that can reduce the risk of heat being conducted to an adjacent battery cell in order to make the cooling property of each battery cell uniform, and a battery pack device using this thermally conductive member. Provided is a thermally conductive member arranged between battery cells when assembling the battery cells into a battery pack, wherein the thermally conductive member includes thermally conductive layers each having a thermal conductivity of 0.5 W/mK or more provided respectively on both sides of a backing layer having a thermal conductivity of less than 0.5 W/mK. Especially, it is preferred that a resin member forming the backing layer has a flexural modulus of 1 GPa or more.