[Technical Field] [0001] The present invention relates to a turbine used in, for instance, a power plant, a chemical plant, a gas plant, a steel plant, or a vessel. Priority is claimed on Japanese Patent Application No. 2011-204138, filed on September 20, 2011, the content of which is incorporated herein by reference. [Background Art] [0002] As a type of steam turbine, steam turbines having a casing, a shaft body (rotor) that is rotatably installed inside the casing, a plurality of turbine vanes that are fixedly disposed on an inner circumference of the casing, and a plurality of turbine blades that are radially installed on the shaft body on a downstream side of the plurality of turbine vanes have been known. In the case of an impulse turbine among these steam turbines, pressure energy of steam is converted into velocity energy by the turbine vanes, and the velocity energy is converted into rotating energy (mechanical energy) by the turbine blades. Further, in the case of a reaction turbine, the pressure energy is converted into velocity energy even inside the turbine blades, and into rotating energy (mechanical energy) by a reaction force with which the steam is spouted out. [0003] In this type of steam turbine, radial clearance is formed between a tip portion of the turbine blade and the casing surrounding the turbine blade to form a flow passage of the steam. Further, the radial clearance is also formed between the tip portion of the turbine vane and the shaft. However, leakage steam passing through the clearance of the tip portion of the turbine blade on the downstream side does not offer a rotating force to the turbine blade. Further, leakage steam passing through the clearance of the tip portion of the turbine vane on the downstream side hardly offers a rotating force to the downstream turbine blade, because the pressure energy of steam is not converted into the velocity energy by the turbine vane. Accordingly, to improve performance of the steam turbine, it is necessary to reduce the amount of the leakage steam passing through the clearance. [0004] In Patent Literature 1 below, there is a proposal for a structure in which the tip portion of the turbine blade are provided with step part whose heights are gradually increased from the axial upstream side to the downstream side, and the casing is provided with seal fins having clearance with respect to the step part. With this configuration, a leakage flow passing through the clearance of the seal fins collides with end edges of the step part which form step faces of the step part, and increases flow resistance. Thereby, the leakage flow rate is reduced. [Citation List] [Patent Literature] [0005] [Patent Literature 1 ] Japanese Unexamined Patent Application, First Publication No. 2006-291967 [Summary of Invention] [Technical Problem] [0006] However, there is great demand for improvement in the performance of the steam turbine, and thus there is a need to further reduce the leakage flow rate. [0007] The present invention has been made in consideration of such circumstances and an object of the present invention is to provide a high-performance turbine capable of further reducing a leakage flow rate. [Solution to Problem] [0008] According to a first aspect of the present invention, a turbine includes blades, and structures that are provided at sides of tips of the blades with a gap and rotate around axes thereof relative to the blades. One of a tip portion of the blade and a portion of the structure which corresponds to the tip portion of the blade includes step part that have a step face that protrudes toward the other, the other is provided with seal fins extending out with respect to the step part and form minute clearance (H) between the step part and the other. The step part facing the seal fins is configured to protrude so that a cavity forming a main vortex and counter vortex being formed by the main vortex are formed on an upstream side of the seal fins, and the cavity is formed so that the axial width dimension (W) and the radial height dimension (D) satisfy Formula (1) below. 0.45