A first score line (430) is provided with a modified groove width portion (435) in which the groove width differs from the groove width of other portions. This modified groove width portion (435) is formed by imparting curvature to a point of a first side surface (430B) and this portion of a second side surface (430C) are respectively distended in an outward direction. To describe further, the modified groove width portion (435) is formed by forming respective recessed portions in the first side surface (430B) and the second side surface (430C). In so doing, rupture of a panel can take place in satisfactory fashion along a score line having a branched portion.