A flat harness winding device includes: a case; a rotor rotatably disposed in the case; a shaft of the case inserted into a hole in the center of the rotor; a plurality of harness split rollers rotatably disposed in the rotor; and a plurality of flat harnesses each of which one end is fixed to the shaft, said flat harnesses independently bent along respective harness split rollers and guided out from an opening of the case allowed to be wound. Harness support rollers are rotatably disposed on the rotor in spaces between the harness split rollers. Curve-shaped harness guide ribs are disposed on the rotor close to an out side in a radial direction of each harness split roller.