maximum temperature and there is a temperature gradient u03b4t towards either end l 2 of the metallic mold and a viscosity of the plastic material has a minimum value at the center 0 of the metallic mold as shown by the curve u03b7 in fig9 in the case when the composite tubing is pressed by utilizing the above described metallic mold air in the composite tubing is gradually forced out of the interior of the tubing so that there is no inclusion of an air bubble in the optical fiber joint experiments have demonstrated that if nylon is utilized as the plastic material the optical fiber joint does not include any air bubbles at a thermal gradient u03b4t in the range of 30 u00b0 70 u00b0 c so that it is possible to obtain highly reliable optical fiber joints it is to be noted that a probability in which air bubbles each having more than 1 mm diameter are generated at a temperature gradient u03b4t is 20 u00b0 c was seven cases per one hundred cases on the other hand in the case where a thermal gradient u03b4t is 70 u00b0 c or more the central part of the metallic mold reaches excessively high temperatutes before the temperature at the ends of the metallic mold rises to or above the melting point of nylon.