PURPOSE: To reduce the outside diameter of the flange of a hub, in a flywheel hub structure.

CONSTITUTION: On the connection side with a hub of a housing 5, five edge pieces 53 which bulge outside from the outer peripheral surface of the housing 5, keeping each equal interval in the outer peripheral direction. On an edge piece 53 are formed. A through hole 54 for the insertion of a tightening bolt with the flange 31 of the hub is formed, and a tapped hole 31 which consists of the continuation of the through holes 54 is formed on the flange 31. Further, a through hole 31a for the insertion of a tightening bolt with a load wheel is formed at each position corresponding to the outside in the radial direction, between the contiguous tapped holes 31b on the flange 31. Since the pitch circle radius R2 of a bolt for tightening between the hub and the load wheel can be set close to the pitch circle radius R1 of the bolt for tightening the hub and the housing 5, the outside diameter of the flange 31 of the hub can be reduced.