To make fluctuation in line diameters of a fiber melted and spun from a base material for optical fibers small and prevent dew formation in a cooling cylinder for cooling the fiber.
A cooling cylinder 1 for a spun fiber is constituted to be divided into an upper half part A and the lower half part B of the cylinder body and so as to have a structure of inner and outer double cylinders made of an inner cylinder 4 and an outer cylinder 5 and bring each of left and right half parts 1L and 1R to be separately openable and closable. During drawing, a gas such as N2 gas, dry air, He gas, etc., is supplied to the inner cylinder 4 and circulating liquids having temperature difference are supplied to the upper half part 5a and the lower half part 5b of the outer cylinder 5. After the drawing is finished, the circulation liquid having normal temperature is supplied in the outer cylinder 5 and a gas having normal temperature is supplied into the inner cylinder 4 to prevent dew formation by bringing the cooling cylinder 1 to normal temperature, then, the left and the right half parts 1L and 1R are opened to left and right.
ODA HIDEKI
KOAIZAWA HISASHI
ORITA NOBUAKI
