To provide a belt manufacturing method by which a molding is performed as sufficiently as possible, and no air bubbles exist inside.

A gas layer 5 is made with a rubber tube 4 for vulcanization positioned outside of a molded object 3 set on a mold 2. When the molded object 3 is heated and pressurized, a time difference while molding is created by making a heated and pressurized fluid flow in outside of the gas layer 5 and the gas in the gas layer flow out. By this time difference a time for residual air to escape is secured.