A cover portion and a front portion have a first cut portion on a surface side of the cover portion and a second cut portion on a surface side of the front portion within a range in which a sealing tape having an adhesive surface is to be put across the cover portion and the front portion in a state in which an upper end opening is closed with the cover portion and an inserting piece is inserted along the inside of the front portion. The first cut portion and the second cut portion have at least two first cuts which are formed in a peeling direction for the sealing tape and at least one second cut which is formed in such a direction as to intersect with the peeling direction and which is in contact with or intersects with the first cuts.