A wind turbine blade apparatus comprising: a wind turbine blade body having a leading edge and a trailing edge and an exterior shape which is formed by a first surface and a second surface disposed opposite to the first surface, wherein a serration portion is disposed on at least a part of the trailing edge of the wind turbine blade body, the serration portion having a saw-teeth shape where a mountain portion and a valley portion are arranged alternately in a blade longitudinal direction, wherein the serration portion is configured as an attachment member attached to at least a part of the trailing edge of the wind turbine blade body so as to cover a trailing-edge side portion of the wind turbine blade body, wherein the attachment member includes: a first-surface side wall surface disposed on the first surface of the wind turbine blade body; and a second-surface side wall surface disposed on the second surface of the wind turbine blade body, the second-surface side wall surface being connected to the first-surface side wall surface via a trailing edge portion, wherein trailing-edge portion sides of the first-surface side wall surface and the second-surface side wall surface are formed into a saw-teeth shape where a mountain portion and a valley portion are arranged alternately, and wherein a chord-directional cross section of the wind turbine blade body along a chord direction is formed to have an airfoil shape at any position in a region from a tip portion of the mountain portion to a deepest portion of the valley portion, wherein the airfoil shape is a shape whose thickness changes continuously in the chord-directional cross section and does not have discontinuous points for which two different tangents can be set, except for the trailing edge and the leading edge, on the outer shape of the chord-directional cross section, and wherein the mountain portions and the valley portions formed on the trailing-edge portion sides of the first-surface side wall surface and the second-surface side wall surface are respectively aligned along the chord direction. a wind turbine blade body having a leading edge and a trailing edge and an exterior shape which is formed by a first surface and a second surface disposed opposite to the first surface, wherein a serration portion is disposed on at least a part of the trailing edge of the wind turbine blade body, the serration portion having a saw-teeth shape where a mountain portion and a valley portion are arranged alternately in a blade longitudinal direction, wherein the serration portion is configured as an attachment member attached to at least a part of the trailing edge of the wind turbine blade body so as to cover a trailing-edge side portion of the wind turbine blade body, wherein the attachment member includes: a first-surface side wall surface disposed on the first surface of the wind turbine blade body; and a second-surface side wall surface disposed on the second surface of the wind turbine blade body, the second-surface side wall surface being connected to the first-surface side wall surface via a trailing edge portion, a first-surface side wall surface disposed on the first surface of the wind turbine blade body; and a second-surface side wall surface disposed on the second surface of the wind turbine blade body, the second-surface side wall surface being connected to the first-surface side wall surface via a trailing edge portion, wherein trailing-edge portion sides of the first-surface side wall surface and the second-surface side wall surface are formed into a saw-teeth shape where a mountain portion and a valley portion are arranged alternately, and wherein a chord-directional cross section of the wind turbine blade body along a chord direction is formed to have an airfoil shape at any position in a region from a tip portion of the mountain portion to a deepest portion of the valley portion, wherein the airfoil shape is a shape whose thickness changes continuously in the chord-directional cross section and does not have discontinuous points for which two different tangents can be set, except for the trailing edge and the leading edge, on the outer shape of the chord-directional cross section, and wherein the mountain portions and the valley portions formed on the trailing-edge portion sides of the first-surface side wall surface and the second-surface side wall surface are respectively aligned along the chord direction.