Turbine

A turbine comprising:

• a blade;
• and
• a structure being provided in a tip portion of the blade with a gap and which configured to rotate relatively shaft axis relative to the blade,
• wherein one of the tip portion of the blade and a portion of the structure corresponding to the tip portion of the blade is provided with a step part having a step face that protrudes toward the other, the other is provided with seal fins extending out with respect to the step part and forming 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 an axial width dimension (W) and a radial height dimension (D) satisfy Formula (1) below.
• 0.45≦D/W≦2.67 (1)