Abstract: A duct possessing structure includes an annular first floor and an annular first roof disposed substantially horizontally, vertical stationary guiding partitions provided radially between the first floor and roof, and a centrally located hollow cylindrical space. A revolving body includes a circular second floor and a circular second roof, and a pair of guide pillar members disposed between the second floor and roof. Each guide pillar member comprises a duct-blocking arcuate outer wall along the outer circumference of the second floor and roof, and a duct-forming curved inner wall connecting both side edges of the arcuate outer wall. The revolving body is disposed within the hollow cylindrical space of the duct possessing structure and is adapted for rotation about an axis coaxial with that of the center axis of the cylindrical space.