An upper panel has a shearing line including a shear-start portion, linear portions extending from opposite ends of the shear-start portion in an equal length, and an arced portion connecting leading edges of the linear portion. The upper panel also has foldable lines and ribs on the upper panel, each of the foldable lines being located at a position inner to and in parallel to each of the linear portions, and each of the ribs extending in parallel with each of the foldable lines. A pull tab has a fixing portion fixed to a portion of the upper panel inner to the shear-start portion. The pull tab has a pressing portion extending out the shearing line so as to press a portion of the upper panel located outside of the shear-start portion when a raising portion of the pull tab is raised. When the pull tab is raised for opening operation, each of the linear portions of a part detached from the upper panel has a sheared edge curled toward the inside of a container, providing safety.