PURPOSE: To enhance bonding strength and resin impregnating characteristics by giving a twist to the coupling parts on both sides of the central part in mutually opposite directions without giving a twist to the central part of the coupling part in the case of forming the coupling part by means of opening the end parts of two glass fiber strands that couple each other in the form of nearly short fibers.

CONSTITUTION: When glass fiber strands 1, 2 are coupled together, the end parts of respective glass fiber strands 1, 2 are opened in the form of nearly short fibers and the coupling part 3 is formed by mixing together these opened fibers In this case, it is arranged that twisting is not given to the central part 4 of the coupling part 3, but twisting is given to the coupling parts on both sides of the central part in mutually opposite directions. And this coupling is formed by inserting respective glass fiber strands 1, 2 into a round-shaped cylindrical passage 6 whose section is in a round shape and having slit 5 for insertion of strands in a state wherein they are laid to overlap each other, the end of the glass fiber strand on one side and the other glass fiber strand are fixed together by a fixture 8, and compressed air is blown in from an air blowing hole that is opened to the round-shaped passage 6 at right angles.