A method for manufacturing a wind turbine blade which is at least partially formed of fiber-reinforced plastic, the method comprising: a fiber drawing step of drawing a plurality of fiber bundles each made of reinforcement fiber from a plurality of bobbins, respectively; a reinforcement fiber sheet formation step of arranging the plurality of fiber bundles drawn from the bobbins next to one another and opening the plurality of fiber bundles so as to overlap adjacent two of the fiber bundles and form a reinforcement fiber sheet from the plurality of fiber bundles; a resin-impregnated fiber sheet formation step of impregnating the reinforcement fiber sheet with a thermohardening resin which is solventless and in a liquid form so as to form a resin-impregnated fiber sheet including the reinforcement fiber and the uncured thermohardening resin; and a molding step of arranging on a mold the resin-impregnated fiber sheet in such a state that the thermohardening resin is uncured and then thermally curing the thermohardening resin of the resin-impregnated fiber sheet on the mold so as to mold the wind turbine blade, the method being characterized in that, in the reinforcement fiber sheet formation step, the plurality of fiber bundles being transferred continuously are opened between an upstream nip part and a downstream nip part each including a pair of rollers (6A, 6B, 8A, 8B) in such a state that the plurality of fiber bundles are nipped between the pair of rollers (6A, 6B, 8A, 8B) of each of the upstream nip part and the downstream nip part, wherein the pair of rollers (6A, 6B) of the upstream nip part rotates at the same circumferential speed as the pair of rollers (8A, 8B) of the downstream nip part, and wherein, in the reinforcement fiber sheet formation step, compressed gas is sprayed from below by at least one nozzle disposed below the fiber bundles between the upstream nip part and the downstream nip part so as to face the plurality of fiber bundles arranged next to one another, and the plurality of fiber bundles are opened by the compressed gas, wherein between the upstream nip part and the downstream nip part a compressed gas discharge unit is provided, the unit being formed by a movable part having the at least one nozzle and a stationary part arranged to oppose the movable part. a fiber drawing step of drawing a plurality of fiber bundles each made of reinforcement fiber from a plurality of bobbins, respectively; a reinforcement fiber sheet formation step of arranging the plurality of fiber bundles drawn from the bobbins next to one another and opening the plurality of fiber bundles so as to overlap adjacent two of the fiber bundles and form a reinforcement fiber sheet from the plurality of fiber bundles; a resin-impregnated fiber sheet formation step of impregnating the reinforcement fiber sheet with a thermohardening resin which is solventless and in a liquid form so as to form a resin-impregnated fiber sheet including the reinforcement fiber and the uncured thermohardening resin; and a molding step of arranging on a mold the resin-impregnated fiber sheet in such a state that the thermohardening resin is uncured and then thermally curing the thermohardening resin of the resin-impregnated fiber sheet on the mold so as to mold the wind turbine blade, the method being characterized in that, in the reinforcement fiber sheet formation step, the plurality of fiber bundles being transferred continuously are opened between an upstream nip part and a downstream nip part each including a pair of rollers (6A, 6B, 8A, 8B) in such a state that the plurality of fiber bundles are nipped between the pair of rollers (6A, 6B, 8A, 8B) of each of the upstream nip part and the downstream nip part, wherein the pair of rollers (6A, 6B) of the upstream nip part rotates at the same circumferential speed as the pair of rollers (8A, 8B) of the downstream nip part, and wherein, in the reinforcement fiber sheet formation step, compressed gas is sprayed from below by at least one nozzle disposed below the fiber bundles between the upstream nip part and the downstream nip part so as to face the plurality of fiber bundles arranged next to one another, and the plurality of fiber bundles are opened by the compressed gas, wherein between the upstream nip part and the downstream nip part a compressed gas discharge unit is provided, the unit being formed by a movable part having the at least one nozzle and a stationary part arranged to oppose the movable part.