Provided is a turbine in which seal fins (15) are provided to a hub shroud (41) to protrude toward a bottom surface (33a) of an annular groove (33), small gaps (H) are formed between the tip ends of the seal fins and the bottom surface of the annular groove, and assuming that the axial distance between a first seal fin (15a) disposed on the front edge side of the hub shroud among the seal fins and a front edge (41a) of the hub shroud is (L) and the axial distance between the front edge of the hub shroud and an inside surface (33b) of the annular groove is (Bu), the first seal fin is disposed to satisfy L/Bu