PURPOSE: To use a connected section protecting tube so that occurrence of disconnection in an optical cable in the vicinity of an optical core line connected section can be prevented even if the tube is used under a condition where the temperature change is relatively large, by integrating the end sections of the tube with the sheaths of the cables to one body by heating and setting the length of the optical core line from the heated section to the connected section at ≥1.5m.

CONSTITUTION: Length of an optical core line 2 from a heated section 2 to an optical core connected section 3 is set at ≥1.5m. At the end sections of optical cables 1 to be connected with each other, the Pb sheaths 10 of the cables 1 are removed and the optical core lines 2 are exposed. The exposed core lines 2 are connected with each other by fusion and an optical core connected section 3 is formed. Then the connected section 3 and the loop-like parts of the core lines 2, namely, the excessive length parts of the core lines 2 are put in a connecting box 4. Outside of the connecting box 4 is covered with a Pb protecting tube 5 having an inner diameter which can house the box 4 and both end sections 5a and 5b of the tube 5 are tightly brought into contact with the Pb sheaths 10 of the optical cable and sections 1a and 1b by a diameter reducing work. Then the tightly contacting parts 5a and 5b are heated by a plumber and heat welded sections 6 and 6 are welded to the Pb sheaths 10 and 10 of the optical cables 1 and 1 so as to form a watertight structure.