A seal structure that seals a gap (Gd) between a first structure (10,51) and a second structure (10,51) which faces the first structure in a radial direction and rotates relative to the first structure about an axis, wherein one of the first structure (10,51) and the second structure (10,51) has base surfaces and a step surface that protrudes toward the other side from the base surfaces, wherein the other of the first structure (10,51) and the second structure (10,51) is provided with: a first fin which extends toward the step surface and forms a first gap (mB) at a space with respect to the step surface, and a second fin that, on a downstream side of the first fin, extends toward the base surface and forms a second gap (mC) at a space with respect to the base surface, characterized in that the other structure of the first structure (10,51) and the second structure (10,51) further comprises a protrusion part that is disposed between the first fin and the second fin and that divides a leak flow which has passed through the first gap (mB), into a first vortex along the first fin and a second vortex along the second fin, the protrusion part includes a reattachment edge that extends in a circumferential direction and causes the leak flow to be reattached thereon, wherein the reattachment edge is provided in a position between a downstream side end part of the step surface and the second fin in the axial direction, and between the step surface and the other structure in the radial direction, and wherein the protrusion part is a member that is connected to an upstream side surface of the second fin and that is of a cylindrical shape concentric with the axis, extending in the circumferential direction between the reattachment edge and the second fin, wherein the reattachment edge is the most upstream side end part of the protrusion and opposes the base surface at the downstream side with respect to the step surface in the radial direction along with the reattachment edge not being in a position where it opposes the step surface in the radial direction. a first fin which extends toward the step surface and forms a first gap (mB) at a space with respect to the step surface, and a second fin that, on a downstream side of the first fin, extends toward the base surface and forms a second gap (mC) at a space with respect to the base surface, characterized in that the other structure of the first structure (10,51) and the second structure (10,51) further comprises a protrusion part that is disposed between the first fin and the second fin and that divides a leak flow which has passed through the first gap (mB), into a first vortex along the first fin and a second vortex along the second fin, the protrusion part includes a reattachment edge that extends in a circumferential direction and causes the leak flow to be reattached thereon, wherein the reattachment edge is provided in a position between a downstream side end part of the step surface and the second fin in the axial direction, and between the step surface and the other structure in the radial direction, and wherein the protrusion part is a member that is connected to an upstream side surface of the second fin and that is of a cylindrical shape concentric with the axis, extending in the circumferential direction between the reattachment edge and the second fin, wherein the reattachment edge is the most upstream side end part of the protrusion and opposes the base surface at the downstream side with respect to the step surface in the radial direction along with the reattachment edge not being in a position where it opposes the step surface in the radial direction.