To provide a manufacturing method of a transmission belt extended in its life by accurately molding cog ridge parts including the joint part in a compressed rubber layer and improving the bending fatigue resistance of the joint part.

In the manufacturing method of the transmission belt wherein a compressed rubber layer 7 and a stretched rubber layer 6 are arranged so as to be adjacent to an adhesive rubber layer 6 in which core wires 3 are embedded and the compressed rubber layer 7 has cog parts 12 wherein cog ridge parts 9a and 9b and cog valley parts 8 are arranged alternately, the joint parts 14a and 35a of the compressed rubber layer 7 are allowed to exist in the region of the cog ridge parts 9a and one of the cog ridge parts 9a including at least the joint parts 14a is molded by a hard mold 20b while the other ridge parts 9b are molded by a mold 20a made of a rubber.