To provide a joining method of a toothed belt for suppressing degradation of meshing of the belt with a pulley caused by shift of a core to cause a large PLD so as to prevent a problem such as jumping, when the toothed belt made of thermoplastic resin material is joined.

In the joining method of a toothed belt, the core 3 extending in a longitudinal direction of the belt is buried in a thermoplastic material, both ends of the toothed belt 1 having tooth forms at predetermined pitches are brought into abutment with each other in a die to fuse and make an endless toothed belt. In the joining method of the toothed belt, the die is provided with recesses and projections meshed with tooth parts of the toothed belt, and is constituted of a first die brought into abutment with a tooth side and a second die brought into abutment with a back side. The first die has a groove 11 in which the toothed belt is fit, and the groove 11 of the first die is deeper than the thickness t of the back of the toothed belt 1, and the second die has a protruded streak part 6 fit to the groove 11 of the first die.