A filling member is interposed between pouch cells of a battery pack, the filling member has a first surface orthogonal to a thickness direction thereof and a second surface opposite to the first surface, θd1 and θd2 defined below satisfy formulas (1) and (2) below, respectively, θp satisfies formula (3) below, and θd1>θd2 holds: (1) θd1≥3.0×10−3 (m2·K)/W, (2) θd2≤8.0×10−3 (m2·K)/W, (3) 0.5 K/W≤θp1≤1000 K/W, and (4) 0.5 K/W≤θp2≤1000 K/W. θd1 is heat transfer resistance when the average temperature of one of the first and second surfaces exceeds 180° C., θd2 is heat transfer resistance when the average temperatures of the first and second surfaces both do not exceed 80° C., θp1 is heat transfer resistance when the average temperature exceeds 180° C., θp2 is heat transfer resistance when the average temperatures do not exceed 80° C.