In the disclosed turbine (1), step sections (52), which have step surfaces (53), are provided on one among a tip section (51) of a blade (50) or an area (11a) of a structure (11) corresponding to the tip section (51), and protrude towards the side of the other one among the tip section (51) and the area (11a) of the structure (11). Seal fins (15) which extend towards the step sections (52) are provided on said other one among the tip section (51) and the area (11a) of the structure (11). Cavities (C) are formed between the tip section (51) of the blade (50) and the area (11a) of the structure (11) between the seal fins (15) and the partition walls (54, 15) which face the seal fins (15) on the upstream side in the direction of the axis of the rotation of the structure (11). Furthermore, the seal fins (15) are equipped with: fin main body sections (16) which form micro gaps (H) between the step sections (52) and the seal fins (15); and space restriction sections (17) which restrict the space of the micro gaps (H) on the upstream side in the direction of the axis of the rotation, and which form small cavities (18) between the cavities (C) and the micro gaps (H). Thus provided is a high performance turbine which the leakage flow rate is reduced.