1. A sealing structure configured to seal a space between an outer peripheral surface of a rotor, the rotor being configured to rotate around an axis of the rotor and an inner peripheral surface of a stator surrounding the rotor from an outer peripheral side of the rotor in a radial direction, the sealing structure comprising:a plurality of step portions on a first surface which is one of the outer peripheral surface of the rotor and the inner peripheral surface of the stator, the plurality of step portions being arranged in an axial direction, each of the plurality of step portions having a stepped surface which faces an upstream side in the axial direction, and protrudes from the first surface toward a second surface which is the other of the outer peripheral surface of the rotor and the inner peripheral surface of the stator; and
a plurality of seal fins on the second surface, each of the plurality of seal fins extending toward a peripheral surface of a corresponding one of the plurality of step portions such that a clearance is defined between each of the plurality of seal fins and the peripheral surface of the corresponding one of the plurality of step portions,
wherein each of the plurality of seal fins includes a tip, a bent point at a position apart from the second surface in the radial direction toward the first surface by a predetermined distance, and a bent portion which is inclined toward the upstream side from the bent point toward the tip on a first surface side,
wherein a cavity is defined between each adjacent pair of the plurality of seal fins and a cavity is defined between one of the plurality of seal fins which is furthest upstream and a side surface of one of the stator and the rotor, the side surface facing toward a downstream side, and
wherein a vertical to horizontal ratio D/W, which is a ratio of a radial size D of each cavity to an axial size W of each cavity, is 1.0.