PURPOSE: To relieve stress to an optical fiber and to prevent the deterio-ration of the characteristics of the optical fiber by reducing the contact area of a low m.p. glass for airtight sealing with the optical fiber.

CONSTITUTION: An optical fiber 1 is allowed to pierce through a metal pipe 4 with a hole of a small diameter for injecting a low m.p. glass, glass pipes 6 used as positioning in common are inserted into the coated parts 2 at the both ends of the optical fiber 1 and the low m.p. glass powder 31 is filled from the small diameter hole of the metal pipe 4 in the state wher the optical fiber 1 is arranged so that the small diameter hole come to the inside of the end face. The metal pipe 4 is then heated with a high-frequency heater 7 to melt the low m.p. glass powder 31.