PURPOSE: To make nearly uniform hot-melt adhesion possible without being influenced by the changes of humidity and temperature of atmosphere and obtain nearly constant bond strength by a method wherein joining yarn is heated up to the required temperature before being passed through molten thermoplastic resin adhesive.

CONSTITUTION: Within a heating box 1, one cop 9 wound by glass fiber yarn 7 and four cops 11 wound by synthetic resin fiber yarn 5 such as polyester resin fiber yarn are provided exchangeably so as to be heated as a whole and at the same time to heat the glass fiber yarn 7 and the synthetic resin fiber yarns 5, which are delivered from the cops 9 and 11 up to the required temperature in order to evaporate moisture adhered to between the respective yarns. After the glass fiber yarn 7 left the cop 9 and the synthetic resin fiber yarns 5 left the cops 11 are delivered outside the heating box 1, the synthetic resin fiber yarns 5 are arranged at positions which nearly quadrisect the peripheral portion round the glass fiber yarn 7 so as to guide the synthetic resin fiber yarns 5 and the glass fiber yarn 7 in a dissolving tank 15 of hot melt 13.