A wind turbine blade comprising: a blade tip part; a blade root part which is connected to a hub of a wind turbine; and an airfoil part which is disposed between the blade tip part and the blade root part and which includes a flatback airfoil with a blunt trailing edge at least in an area in a longitudinal direction of the wind turbine blade where a blade thickness ratio X is not less than 40% and not greater than 50%, X indicating a ratio of a maximum thickness t MAX to a chord length Lc, wherein, in the area where the blade thickness ratio X is not less than 40% and not greater than 50%, the flatback airfoil has a trailing edge thickness ratio Y which is not less than 5% and not greater than Y H %, Y indicating a ratio of a trailing edge thickness t TE to the maximum thickness t MAX, wherein Y H has a relationship of Y H =1.5X-30 with the blade thickness ratio X, and wherein, in such a case that a relationship T≤-5S+5 is satisfied where S is a parameter expressed by S= L CMAX /R, L CMAX being a maximum chord length, R being a rotation radius of the wind turbine blade and T is a parameter expressed by T=RΩ*Lr/R, RΩ being a circumferential speed of the blade tip part at the time of wind speed V 2, Lr being a blade tip representative chord length, V 2 being the wind speed when the rotational speed of the wind turbine reaches a rated rotational speed, the trailing edge thickness ratio Y of the flatback airfoil has a relationship of X-20≤Y≤1.5X-30 in the area where the blade thickness ratio X is not less than 40% and not greater than 50%, wherein, in such a case that a relationship T≥-5S+6 is satisfied, the trailing edge thickness ratio Y of the flatback airfoil is expressed by 5≤Y≤0.5X-5 in the area where the blade thickness ratio X is not less than 40% and not greater than 60%, wherein, in such a case that a relationship -5S+5<T<-5S+6 is satisfied, the trailing edge thickness ratio Y of the flatback airfoil is expressed by 0.5X-5<Y<X-20 in the area where the blade thickness ratio X is not less than 40% and not greater than 50%, and wherein the blade tip representative chord length is a chord length at a blade radius position r where a ratio of the blade radius position r to the rotation radius R is 0.8. a blade tip part; a blade root part which is connected to a hub of a wind turbine; and an airfoil part which is disposed between the blade tip part and the blade root part and which includes a flatback airfoil with a blunt trailing edge at least in an area in a longitudinal direction of the wind turbine blade where a blade thickness ratio X is not less than 40% and not greater than 50%, X indicating a ratio of a maximum thickness t MAX to a chord length Lc, wherein, in the area where the blade thickness ratio X is not less than 40% and not greater than 50%, the flatback airfoil has a trailing edge thickness ratio Y which is not less than 5% and not greater than Y H %, Y indicating a ratio of a trailing edge thickness t TE to the maximum thickness t MAX, wherein Y H has a relationship of Y H =1.5X-30 with the blade thickness ratio X, and wherein, in such a case that a relationship T≤-5S+5 is satisfied where S is a parameter expressed by S= L CMAX /R, L CMAX being a maximum chord length, R being a rotation radius of the wind turbine blade and T is a parameter expressed by T=RΩ*Lr/R, RΩ being a circumferential speed of the blade tip part at the time of wind speed V 2, Lr being a blade tip representative chord length, V 2 being the wind speed when the rotational speed of the wind turbine reaches a rated rotational speed, the trailing edge thickness ratio Y of the flatback airfoil has a relationship of X-20≤Y≤1.5X-30 in the area where the blade thickness ratio X is not less than 40% and not greater than 50%, wherein, in such a case that a relationship T≥-5S+6 is satisfied, the trailing edge thickness ratio Y of the flatback airfoil is expressed by 5≤Y≤0.5X-5 in the area where the blade thickness ratio X is not less than 40% and not greater than 60%, wherein, in such a case that a relationship -5S+5<T<-5S+6 is satisfied, the trailing edge thickness ratio Y of the flatback airfoil is expressed by 0.5X-5<Y<X-20 in the area where the blade thickness ratio X is not less than 40% and not greater than 50%, and wherein the blade tip representative chord length is a chord length at a blade radius position r where a ratio of the blade radius position r to the rotation radius R is 0.8.