A fluid seal structure of a heat engine including a steam turbine includes an inner case which houses a turbine rotor rotatably inside, an outer case which houses the inner case and forms a space through which a fluid can flow between the outer case and an exterior surface of the inner case, a protruding portion protruding into the space from one of the exterior surface of the inner case or an interior surface of the outer case, and a partitioning plate extending into the space from the other one of the exterior surface or the interior surface and being formed annularly in a circumferential direction of the surface. The partitioning plate partitions the space into first and second space sections, and is flexurally deformable in the axial direction of the turbine rotor by an internal fluid pressure difference between the first and second space sections.