A belt member is formed with a seam portion by overlapping and adhering both longitudinal end portions thereof to constitute an endless belt stretched and circulated by a rotative, first stretching member and a second stretching member. A length of the seam portion is no less than a length between a first point at which the endless belt is separated from one of the first stretching member and the second stretching member and a second point at which the endless belt is brought into contact with the other one of the first stretching member and the second stretching member.