The turbine is provided with a blade (50) and a structure body (11) which rotates relatively with respect to the blade (50). A stepped part (52A) having a step surface (53A) is installed at one of the leading end of the blade (50) and the structure body (11) corresponding to the leading end thereof, while a seal fin (15A) which extends toward the stepped part (52A) to form a small space (H) is installed at the other of them. A cavity (C1) is formed between the blade (50) and the structure body (11) and also between the seal fin (15A) and the partition wall which faces thereto in the rotating shaft direction of the structure body (11) on the upstream side. When a distance of the cavity (C1) between the partition wall and the seal fin (15A) is given as a cavity width (W), and a distance between the seal fin (15A) and the end edge (55) of the stepped part (52A) in the rotating shaft direction on the upstream side is given as (L), at least one of the distances (L) satisfies the following formula (1). 0.7H≦L≦0.3W  (1).