An automatic zipper device having a pair of bevel gears for biting and joining joint end portions of a sheet member wound around a periphery of the molding drum and having a plurality of magnet attractive metallic cords, and the device is provided with an inclined portion following cylinder which is attached to a lateral movement device laterally moving on the molding drum so as to be laterally movable and which adjusts a height of the bevel gear in accordance with an inclined surface of the sheet member to be joined, a clamping cylinder for adjusting a bitten amount of the bevel gears, and introduction magnet rollers for taking in the sheet member, and a member contact position stabilizing magnet for maintaining a constant position of the sheet member so that the sheet member to be joined can be brought into contact with the bevel gear at a constant angle.