PURPOSE: To make an optical fiber core run along a groove, and to align and fix it, by using a tensile strength body having a groove in the longitudinal direction, as a tensile strength body.

CONSTITUTION: An internal tube 12 having a heat solubility is fitted and inserted to optical fibers 11, 11' which have been welded and connected at a point A, and also on its tube, an external tube of a heat contractibility having a tensile strength body 13 is fitted and inserted. This tensile strength body 13 is formed by arranging in parallel two pieces of round steel wires 15, 15' and welding them, and a groove 13a is formed in the longitudinal direction. The internal tube 12 and the external tube 14 are formed to a length for covering core 16, 16' parts which have coated the optical fibers 11, 11', and before connecting the optical fibers, one optical fiber is inserted through in advance, and after welding and connecting them, they are set to a position as shown in the figure. When they are heated, the internal tube 12 is melted, and the external tube 14 is contracted. Next, the molten internal tube 12 wraps in the optical fibers 11, 11', and also the external tube 14 presses the cores 16, 16' against the groove 13a made by two pieces of steel wires 15, 15' and aligns them.