PURPOSE: To obtain an infrared-ray optical fiber with uniform nature by using liquid clad material made of high polymer material as a pressure medium, and coating the outer surface of the linear core of an optical fiber in a container with a clad and discharging it from a nozzle.

CONSTITUTION: An optical fiber unit C consists of conical fiber base material A and clad material B of high polymer material covering it. A ram 5 is moved back to the rear end of a unit supply part 4d and the unit C is supplied in front of it; and the unit C is charged in the container 4 by a cylinder 6 and the nozzle 4c is blocked at the conical part. The container 4 is heated by a heater 9 to liquefy the clad material B and also soften the fiber base material A. Controllers 8 and 11 hold the temperature and pressure of the liquid clad material B constant and the outer surface of the linear core (a) is coated with the clad (b) and discharged from the nozzle 4c. Thus, the infrared-ray optical fiber with uniform nature is obtained.