A film cooling structure comprising: a wall part: having an outer surface and an inner surface and extending forward and rearward; a cooling hole penetrating through the wall part, including an inlet opening to the inner surface and an outlet opening to the outer surface, and being inclined such that the outlet is positioned rearward of the inlet; wherein the cooling hole includes a straight-tube part having the inlet, and a diffuser part connecting with the straight-tube part and having the outlet, the straight-tube part is positioned inside the diffuser part on a projection plane of the cooling hole orthogonal to an extending direction of the cooling hole, characterized in that the diffuser part includes: a flat surface; a curved surface curved rearward and forming a channel cross section together with the flat surface, the channel cross section having a semicircular or semi-elliptical shape larger than that of the straight-tube part; a first section in which an area of the channel cross section increases as the channel cross section approaches the outlet of the cooling hole; and a second section in which an area of the channel cross section increases at an increase rate or is constant as the channel cross section approaches the outlet of the cooling hole, the second section extending from the first section toward the outlet of the cooling hole, and the increase rate being smaller than that in the first section, and the diffuser part has a length along the flat surface on the projection plane equal to or twice greater than a length of the diffuser part along a direction orthogonal to the flat surface on the projection plane. a wall part: having an outer surface and an inner surface and extending forward and rearward; a cooling hole penetrating through the wall part, including an inlet opening to the inner surface and an outlet opening to the outer surface, and being inclined such that the outlet is positioned rearward of the inlet; wherein the cooling hole includes a straight-tube part having the inlet, and a diffuser part connecting with the straight-tube part and having the outlet, the straight-tube part is positioned inside the diffuser part on a projection plane of the cooling hole orthogonal to an extending direction of the cooling hole, characterized in that the diffuser part includes: a flat surface; a curved surface curved rearward and forming a channel cross section together with the flat surface, the channel cross section having a semicircular or semi-elliptical shape larger than that of the straight-tube part; a first section in which an area of the channel cross section increases as the channel cross section approaches the outlet of the cooling hole; and a second section in which an area of the channel cross section increases at an increase rate or is constant as the channel cross section approaches the outlet of the cooling hole, the second section extending from the first section toward the outlet of the cooling hole, and the increase rate being smaller than that in the first section, and a flat surface; a curved surface curved rearward and forming a channel cross section together with the flat surface, the channel cross section having a semicircular or semi-elliptical shape larger than that of the straight-tube part; a first section in which an area of the channel cross section increases as the channel cross section approaches the outlet of the cooling hole; and a second section in which an area of the channel cross section increases at an increase rate or is constant as the channel cross section approaches the outlet of the cooling hole, the second section extending from the first section toward the outlet of the cooling hole, and the increase rate being smaller than that in the first section, and the diffuser part has a length along the flat surface on the projection plane equal to or twice greater than a length of the diffuser part along a direction orthogonal to the flat surface on the projection plane.