A fluid seal structure of a heat engine including a steam turbine is provided with: an inner case for rotatably accommodating a turbine rotor within the interior thereof; an outer case for accommodating the inner case and forming a space section through which a fluid can flow between the outer case and the outer surface of the inner case; a protruding section that extends into the space section from either of the outer surface of the inner case or the inner surface of the outer case, the protruding section being formed into an annular shape in the circumferential direction of the surface; and a partitioning plate that extends into the space section from the other of the outer surface and the inner surface, the partitioning plate being formed into an annular shape, and partitions the space section into a first space section on one side of the turbine rotor in the axial direction and a second space section on the other side, the partitioning plate being able to be flexurally deformed in the axial direction of the turbine rotor by the internal fluid pressure difference between the first space section and the second space section.