Abstract: A driving force transmission device includes drive rollers, roller support portions, and a roller arrangement portion. The drive rollers transmit a driving force to a main wheel. The roller support portions rotatably support the drive rollers. The roller arrangement portion includes a base, an outer frame, and an inner frame. The base expands radially outward with respect to a rotation axis. The outer frame extends in a circumferential direction around the rotation axis and protrudes from the base in an axial direction along the rotation axis. The inner frame is radially inside the outer frame and protrudes from the base in the axial direction. The roller support portions are fitted between the inner circumferential surface of the outer frame and the outer circumferential surface of the inner frame.