Seal structure of wire drawing furnace for optical fiber, and production method for optical fiber

A manufacturing method of an optical fiber, comprising:

• drawing the optical fiber by using a seal structure of a wire drawing furnace for the optical fiber for sealing a gap between an upper end opening portion of the wire drawing furnace for the optical fiber and a glass base material for the optical fiber inserted into a furnace core tube from the upper end opening portion, wherein the seal structure includes:
• a plurality of blade members that is circumferentially provided to abut on a circumferential side surface of the glass base material for the optical fiber;
• a guide member that is provided around a periphery of the plurality of blade members and for linearly sliding the plurality of blade members toward the circumferential side surface of the glass base material for the optical fiber;
• and a supply and discharge port for supplying and discharging a gas that causes the blade member to move in a radial direction of the glass base material for the optical fiber;
• and
• providing a center of gravity of the plurality of blade members at a location behind a tip surface of the guide member, from when a tip surface of the blade member is farthest away from the side surface of the glass base material until the tip surface thereof abuts on the side surface of the glass base material, at all times during drawing of the optical fiber.