To provide a belt transmission device that can enhance the fatigue strength of a steel belt and improve durability by piling up an internal belt and an exterior belt.

The belt transmission device includes: a steel belt 10; and a pulley 21 that hoists the steel belt 10. The steel belt 10 relatively movably laminates an internal belt 11 and an exterior belt 12 in the travel direction. Thicknesses of the internal belt 11 and the exterior belt 12 are set so that bending stress when they are wound around the pulley 21 may be lower than an yield point or proof stress.