To provide a transmission belt and its manufacturing method having superior workability, abrasion resistance and durability.

The synchronous belt 1 has a belt main body composed of a plurality of tooth portions 2 along the belt longitudinal direction and a back portion 4 in which a core wire 3 is buried, and tooth cloth 5 is stuck to a surface of the tooth portion 2 when necessary. The tooth portion 2 is composed of an organic peroxide bridged matter of a rubber composition prepared by blending silica of 10-100 pts.wt. and organic peroxide of 1-8 pts.wt. of organic peroxide to a rubber component of 100 pts.wt. including nitrile hydride rubber, and further blending vinyl triethoxysilane oligomer therein.