A trailing edge side panel configured to be attached to a trailing edge side of a wind turbine blade of a wind turbine rotor, the trailing edge side panel comprising: a panel suction surface which is disposed adjacent to a suction surface of the wind turbine blade in a state where the trailing edge side panel is attached to the trailing edge side of the wind turbine blade; a panel pressure surface which is disposed adjacent to a pressure surface of the wind turbine blade in the state where the trailing edge side panel is attached to the trailing edge side of the wind turbine blade; and a panel trailing edge where the panel suction surface and the panel pressure surface intersect with each other, wherein the panel trailing edge has serrations, and characterized in that in the state where the trailing edge side panel is attached to the trailing edge side of the wind turbine blade, in a cross section, orthogonal to a blade longitudinal direction, of at least a partial area in the blade longitudinal direction, the panel trailing edge is positioned on a suction surface side of a first airfoil defined by a contour of the wind turbine blade with respect to an extension line that virtually extends from the trailing edge of the wind turbine blade as an extension of a camber line of the first airfoil, and a camber line of a second airfoil defined by an entire contour of the wind turbine blade and the trailing edge side panel has an S shape defined by a first curved portion and a second curved portion, the first curved portion being positioned on a leading edge side of the wind turbine blade and protruding toward a suction side, the second curved portion being positioned on a panel trailing edge side with respect to the first curved portion and protruding toward a pressure side. a panel suction surface which is disposed adjacent to a suction surface of the wind turbine blade in a state where the trailing edge side panel is attached to the trailing edge side of the wind turbine blade; a panel pressure surface which is disposed adjacent to a pressure surface of the wind turbine blade in the state where the trailing edge side panel is attached to the trailing edge side of the wind turbine blade; and a panel trailing edge where the panel suction surface and the panel pressure surface intersect with each other, wherein the panel trailing edge has serrations, and characterized in that in the state where the trailing edge side panel is attached to the trailing edge side of the wind turbine blade, in a cross section, orthogonal to a blade longitudinal direction, of at least a partial area in the blade longitudinal direction, the panel trailing edge is positioned on a suction surface side of a first airfoil defined by a contour of the wind turbine blade with respect to an extension line that virtually extends from the trailing edge of the wind turbine blade as an extension of a camber line of the first airfoil, and a camber line of a second airfoil defined by an entire contour of the wind turbine blade and the trailing edge side panel has an S shape defined by a first curved portion and a second curved portion, the first curved portion being positioned on a leading edge side of the wind turbine blade and protruding toward a suction side, the second curved portion being positioned on a panel trailing edge side with respect to the first curved portion and protruding toward a pressure side. in a cross section, orthogonal to a blade longitudinal direction, of at least a partial area in the blade longitudinal direction, the panel trailing edge is positioned on a suction surface side of a first airfoil defined by a contour of the wind turbine blade with respect to an extension line that virtually extends from the trailing edge of the wind turbine blade as an extension of a camber line of the first airfoil, and a camber line of a second airfoil defined by an entire contour of the wind turbine blade and the trailing edge side panel has an S shape defined by a first curved portion and a second curved portion, the first curved portion being positioned on a leading edge side of the wind turbine blade and protruding toward a suction side, the second curved portion being positioned on a panel trailing edge side with respect to the first curved portion and protruding toward a pressure side.