A turbine comprising: a blade (50;40); and a structure (11;30) spaced apart from a tip side of the blade (50;40) by a space and relatively rotatable to each other around a rotation axis of a shaft body, wherein, at one of a part of a tip part of the blade (50;40) and a part of the structure (11;30) corresponding to the tip part of the blade (50;40), at least two stepped parts are formed which protrude toward the other one of the part of the tip part of the blade (50;40) and the part of the structure (11;30), wherein the stepped parts have first step surfaces (52,52A-52J) facing to the other one of the part of the tip part of the blade (50;40) and the part of the structure (11;30) and second step surfaces (53,53A-53J) forming the step differences of the first step surfaces (52,52A-52J), wherein a seal fin extending toward the respective stepped part is provided at the other part of the tip part of the blade (50;40) and the part of the structure (11;30), wherein a cavity is formed between the tip part of the blade (50;40) and the part of the structure (11;30) and between the seal fin and a partition wall facing to the seal fin on an upstream side of the structure (11;30) in a direction of the rotation axis, and wherein the seal fin comprises: a fin main body part (16;16A) that forms a small space between the fin main body part (16;16A) and the stepped part, and a space restriction part (17;17A;17B) that restricts a space of the small space on the upstream side in the direction of the rotation axis and forms a small cavity between the cavity and the small space, wherein the space restriction part (17;17A;17B) has a wall surface (17a;17b;17d) in the direction of the rotation axis extending from the fin main body part (17;17A) toward the upstream side in the direction of the rotation axis and facing one of the first step surfaces (52,52A-52J) of the stepped part, and wherein the small cavity is formed between the wall surface (17a;17b;17d) in the direction of the rotation axis and the one of the first step surfaces (52,52A-52J) such that a separation vortex is generated in operation in the small cavity that turns in a direction opposite to a turning direction of a primary vortex generated in the cavity. a blade (50;40); and a structure (11;30) spaced apart from a tip side of the blade (50;40) by a space and relatively rotatable to each other around a rotation axis of a shaft body, wherein, at one of a part of a tip part of the blade (50;40) and a part of the structure (11;30) corresponding to the tip part of the blade (50;40), at least two stepped parts are formed which protrude toward the other one of the part of the tip part of the blade (50;40) and the part of the structure (11;30), wherein the stepped parts have first step surfaces (52,52A-52J) facing to the other one of the part of the tip part of the blade (50;40) and the part of the structure (11;30) and second step surfaces (53,53A-53J) forming the step differences of the first step surfaces (52,52A-52J), wherein a seal fin extending toward the respective stepped part is provided at the other part of the tip part of the blade (50;40) and the part of the structure (11;30), wherein a cavity is formed between the tip part of the blade (50;40) and the part of the structure (11;30) and between the seal fin and a partition wall facing to the seal fin on an upstream side of the structure (11;30) in a direction of the rotation axis, and wherein the seal fin comprises: a fin main body part (16;16A) that forms a small space between the fin main body part (16;16A) and the stepped part, and a space restriction part (17;17A;17B) that restricts a space of the small space on the upstream side in the direction of the rotation axis and forms a small cavity between the cavity and the small space, a fin main body part (16;16A) that forms a small space between the fin main body part (16;16A) and the stepped part, and a space restriction part (17;17A;17B) that restricts a space of the small space on the upstream side in the direction of the rotation axis and forms a small cavity between the cavity and the small space, wherein the space restriction part (17;17A;17B) has a wall surface (17a;17b;17d) in the direction of the rotation axis extending from the fin main body part (17;17A) toward the upstream side in the direction of the rotation axis and facing one of the first step surfaces (52,52A-52J) of the stepped part, and wherein the small cavity is formed between the wall surface (17a;17b;17d) in the direction of the rotation axis and the one of the first step surfaces (52,52A-52J) such that a separation vortex is generated in operation in the small cavity that turns in a direction opposite to a turning direction of a primary vortex generated in the cavity.