To provide a belt transmission device capable of increasing output, restraining axial dislocation of a driven shaft, improving the belt service life, and miniaturizing a motor.

This belt transmission device 1 has the driven shaft 2, a first motor 3, a transmission belt 4, a second motor 5 and a torque transmission part 6. The first motor 3 has a first driving shaft 31 supported in parallel by the driving shaft 2. The second motor 5 has a second driving shaft 51 supported in parallel by the driving shaft 2. The transmission belt 4 is extended between the driven shaft 2 and the first driving shaft 31. A parallel surface passing through the axis of the driving shaft 2 orthogonally to a plane passing through the axis of the driven shaft 2 and the first driving shaft 31, is set as a first boundary, and the side for arranging the first driving shaft 31 among an area divided by the first boundary, is set as a first area. The torque transmission part 6 abuts on the first area side among an outer peripheral surface in the radial direction of the driven shaft 2, and transmits rotation of the second driving shaft 51 to the driven shaft 2.