A wind turbine blade comprising a vortex generator including a plurality of fins, wherein the vortex generator includes: a plurality of fin sets each of which is formed by a pair of the fins (12A,12B) arranged such that the suction surfaces of the pair of fins (12A,12B) face each other; and a platform portion on which the pair of the fins (12A,12B) of said each of the plurality of fin sets are provided, the platform portion having a circular shape in a top view, wherein the fins (12A,12B) of each of the plurality of fin sets are arranged on the platform portion such that extension lines (l CA,l CB) of the fin chords form a predetermined angle with a line (L H) along a chordwise direction of the wind turbine blade such that a gap between the suction surfaces of the pair of fins (12A,12B) widens toward a side of a trailing edge of the wind turbine blade, wherein a height H in m of each of the fins and a maximum chord length C max in m of the wind turbine blade satisfy 0.3×10 -2 ≤H/C max ≤0.9×10 -2, wherein the height H of the fins satisfies H≥0.1δ 1, provided that δ 1 is a boundary layer thickness at a rated tip speed ratio in a blade degradation state, and wherein the height H of the fins satisfies H≤δ 2, provided that δ 2 is a boundary layer thickness at an optimum tip speed ratio in a blade normal state in which a surface of the wind turbine blade is smoother than in the blade degradation state, wherein the fins are configured such that: an angle (Θ) of the fin chord with reference to an inflow direction of wind is not smaller than 12 degrees and not greater than 18 degrees, a ratio L/H of a chord length L of a fin root to the height H of the fins satisfies 2.0≤L/H≤4.0, and a ratio S/H of an interval S between trailing edges of the pair of fins (12A,12B) to the height H of the pair of fins (12A,12B) satisfies 2.5≤S/H≤5.0, and wherein the vortex generator is arranged such that a ratio Z/S of an arrangement pitch Z of adjacent two of the fin sets to an interval S between the trailing edges of the pair of fins (12A,12B) is not smaller than 1.5 and not greater than 3.0. a plurality of fin sets each of which is formed by a pair of the fins (12A,12B) arranged such that the suction surfaces of the pair of fins (12A,12B) face each other; and a platform portion on which the pair of the fins (12A,12B) of said each of the plurality of fin sets are provided, the platform portion having a circular shape in a top view, wherein the fins (12A,12B) of each of the plurality of fin sets are arranged on the platform portion such that extension lines (l CA,l CB) of the fin chords form a predetermined angle with a line (L H) along a chordwise direction of the wind turbine blade such that a gap between the suction surfaces of the pair of fins (12A,12B) widens toward a side of a trailing edge of the wind turbine blade, wherein a height H in m of each of the fins and a maximum chord length C max in m of the wind turbine blade satisfy 0.3×10 -2 ≤H/C max ≤0.9×10 -2, wherein the height H of the fins satisfies H≥0.1δ 1, provided that δ 1 is a boundary layer thickness at a rated tip speed ratio in a blade degradation state, and wherein the height H of the fins satisfies H≤δ 2, provided that δ 2 is a boundary layer thickness at an optimum tip speed ratio in a blade normal state in which a surface of the wind turbine blade is smoother than in the blade degradation state, wherein the fins are configured such that: an angle (Θ) of the fin chord with reference to an inflow direction of wind is not smaller than 12 degrees and not greater than 18 degrees, a ratio L/H of a chord length L of a fin root to the height H of the fins satisfies 2.0≤L/H≤4.0, and a ratio S/H of an interval S between trailing edges of the pair of fins (12A,12B) to the height H of the pair of fins (12A,12B) satisfies 2.5≤S/H≤5.0, and wherein the vortex generator is arranged such that a ratio Z/S of an arrangement pitch Z of adjacent two of the fin sets to an interval S between the trailing edges of the pair of fins (12A,12B) is not smaller than 1.5 and not greater than 3.0. an angle (Θ) of the fin chord with reference to an inflow direction of wind is not smaller than 12 degrees and not greater than 18 degrees, a ratio L/H of a chord length L of a fin root to the height H of the fins satisfies 2.0≤L/H≤4.0, and a ratio S/H of an interval S between trailing edges of the pair of fins (12A,12B) to the height H of the pair of fins (12A,12B) satisfies 2.5≤S/H≤5.0, and wherein the vortex generator is arranged such that a ratio Z/S of an arrangement pitch Z of adjacent two of the fin sets to an interval S between the trailing edges of the pair of fins (12A,12B) is not smaller than 1.5 and not greater than 3.0.