To restrict deterioration of non-circularity in a constricted portion of an optical fiber preform, provided is an optical fiber preform manufacturing method including introducing inert gas along a surface of the optical fiber preform, causing the inert gas to circulate around the optical fiber preform with an axis of the optical fiber preform in a longitudinal direction as a center, and heating the optical fiber preform in an inert gas environment. In this manufacturing method, the inert gas may be introduced in a direction parallel to a tangent line of the optical fiber preform in a plane orthogonal to the axis.

A seal structure for an optical fiber drawing furnace is for plugging a gap between an upper end opening of the fiber furnace, and an optical fiber glass preform wherein a seed rod and a taper portion are present in an upper portion thereof. The seal structure comprises a first cap member engaging the seed rod of the glass preform; a second cap member covering the taper portion of the glass preform and the first cap member; a spacer member disposed between the first and second cap member, supporting the second cap member, adjusting, via a positional adjustment structure, the height position of the second cap member in the axial direction, and causing the lower extremity of the second cap member to be at a position close to the taper portion; and a seal member which seals between the upper end opening and the glass preform and/or second cap member.

A seal structure for an optical fiber drawing furnace is for plugging a gap between an upper end opening of the fiber furnace, and an optical fiber glass preform wherein a seed rod and a taper portion are present in an upper portion thereof. The seal structure comprises a first cap member engaging the seed rod of the glass preform; a second cap member covering the taper portion of the glass preform and the first cap member; a spacer member disposed between the first and second cap member, supporting the second cap member, adjusting, via a positional adjustment structure, the height position of the second cap member in the axial direction, and causing the lower extremity of the second cap member to be at a position close to the taper portion; and a seal member which seals between the upper end opening and the glass preform and/or second cap member.