A seal mechanism is provided which comprises a structure capable of accommodating radial expansions of turbine rotor without increasing an axial size thereof and providing a reliable sealing property for the turbine rotor for a long time. The seal mechanism comprises a labyrinth seal 91, 93 defined by a projection 87 provided on the turbine rotor 11B, an outer member 67 provided radially outward of the projection and supported for displacement in a radial direction, and an inner member 69 provided radially inward of the projection and supported for displacement in the radial direction. A thermal expansion rate of the inner member is greater than that of the outer member.