A gas releasing valve 50 is formed by a first valve body portion 51 and a second valve body portion 52. The end of the first valve body portion 51 and the end of the second valve body portion 52 both located at a released gas downstream side are joined to a gas releasing wall 41 and a gas releasing wall 42, respectively, with a corresponding one of a first downstream end joint portion 53 and a second downstream end joint portion 54. A flexible portion 58 is formed in a part of each one of the valve body portions 51, 52 in the vicinity of and on a released gas upstream side of the corresponding one of the downstream end joint portions 53, 54. A front portion of the first valve body portion 51 and a front portion of the second valve body portion 52 are joined to at least the gas releasing wall 41 and the gas releasing wall 42, respectively, with a first edge joint portion 55. The first edge joint portion 55 functions also as a wall joint portion 57, which joins at least the gas releasing walls 41, 42 to each other. A rear part of the first valve body portion 51 and a rear part of the second valve body portion 52 are joined to each other with a second edge joint portion 56, which extends toward the released gas upstream side. An auxiliary joint portion 61, which joins the valve body portions 51, 52 to each other, is arranged between the first edge joint portion 55 and the second edge joint portion 56 and in the vicinity of the second edge joint portion 56. The auxiliary joint portion 61 functions also as a flex-resistant portion 59, which extends toward the released gas upstream side and is less flexible than the flexible portion 58.