A back-film structure may include a first back-film portion which serves as a first circle upon rolling and a second back-film portion arranged side by side with the first back-film portion in the rolling direction, wherein a thickness of the first back-film portion increases in the rolling direction. This solution can ensure that the inner surface of the tail end of the first back-film portion is located as close as possible to the inner surface of the head end of the first back-film portion after the rolling.