PURPOSE: To ensure sufficient tension strength and bending fatigue level by forming a steel code of a plurality of filaments extending in a vertical direction of a transmission belt, and also forming a cross section of the steel code in a layer structure having a central core of the filaments.

CONSTITUTION: A toothed transmission belt 1 is reinforced by steel cords 2. The steel codes 2 lies essentially in the longitudinal direction of the transmission belt, and is formed of four and at most twenty-eight filaments having a diameter of between 0.04 mm and 0.20 mm. The layer structure is formed of a central core of one or more filaments surrounded by at least one of filament layers 24, 26. In the steel codes 2, a plurality of steel codes 2I to 2VI are arranged in parallel with each other in a plane surface. The transmission belt 1 is made of polymer such as polyurethane, and the filaments 22, 24, 26 are covered by anticorrosion coating.