To obtain stable differential pressure and prevent cracks and cuts on a sheet strip while continuously feeding a long sheet strip in and out of multiple differential pressure chambers in a differential pressure seal device.

In a pressure range of an initial viscous flow, a seal structure is comprised of a single guide roll controlling a winding-up guide of a long sheet strip, a seal attachment in which the seal attachment is provided to surround the periphery of the wound-up long sheet strip, a part of one opposing face approaches to a surface of the sheet strip on the guide roll and is formed with a shape along the sheet strip, a part of other opposing face approaches to an outer periphery of the guide roll and is formed with a shape along the outer periphery of the guide roll, and a side structure keeping both sides facing to the guide roll airtight is provided. In a pressure range with an intermediate flow and a molecular flow, the seal structure has a long slit-shaped gap.