Abstract: Battery element 2 is sandwiched between and surrounded by casing films 4, 5 each having a thermo-fusing resin layer, and is sealed by thermally fused region 6 formed by thermally fusing around the overall periphery. Cross-link structure portion 13 is formed in part of thermally fused region 6 by cross-linking casing film 5, and gas release chamber 12 is formed with its leading end positioned in cross-link structure portion 13. Gas release chamber 12 is a portion which is surrounded by thermally fused region 6 along its periphery, and in which casing films 4, 5 are not thermally fused. Tube 14 which is open at both ends is connected to gas release chamber 12, while sandwiched between casing films 4, 5, with its leading end positioned in gas release chamber 12.