A rotor blade which has a chord length gradually increased from a blade root to a maximum chord length portion, the maximum chord length portion is a base portion of a blade end portion, characterized in that the rotor blade comprises: - a leading edge that faces a direction of rotation and extends from the blade root to a tip portion of the blade end portion, and, as seen from a side of the rotor blade, the leading edge has a thickness that is greatest at the blade root and gradually and continuously decreases from the blade root to the tip portion; - a front surface gradually inclined in a direction of a back surface from the blade root to the maximum chord length portion such that an interval between the front and back surfaces (1D, 1E) is continuously decreased from the blade root to the maximum chord length portion as seen from the side of the rotor blade; and - an inclined portion formed of a blade end portion which is inclined in a direction of the front surface from the maximum chord length portion, wherein - the front surface extends from the leading edge to a trailing edge of the rotor blade, the trailing edge being positioned behind the leading edge in the direction of rotation, the front surface is oriented, at the blade root, in an oblique direction relative to the direction of rotation such that, from the leading edge to the trailing edge, the front surface is angled toward the back surface, and from the blade root to the maximum chord length portion, a crossing angle of an orientation direction of the front surface relative to the direction of rotation gradually decreases, and wherein - the back surface is gradually inclined in a direction of the front surface from the blade root to the maximum chord length portion according to the continuous decrease of the interval between the front and back surfaces (1D, 1E) from the blade root to the maximum chord length portion in the side view. - a leading edge that faces a direction of rotation and extends from the blade root to a tip portion of the blade end portion, and, as seen from a side of the rotor blade, the leading edge has a thickness that is greatest at the blade root and gradually and continuously decreases from the blade root to the tip portion; - a front surface gradually inclined in a direction of a back surface from the blade root to the maximum chord length portion such that an interval between the front and back surfaces (1D, 1E) is continuously decreased from the blade root to the maximum chord length portion as seen from the side of the rotor blade; and - an inclined portion formed of a blade end portion which is inclined in a direction of the front surface from the maximum chord length portion, wherein - the front surface extends from the leading edge to a trailing edge of the rotor blade, the trailing edge being positioned behind the leading edge in the direction of rotation, the front surface is oriented, at the blade root, in an oblique direction relative to the direction of rotation such that, from the leading edge to the trailing edge, the front surface is angled toward the back surface, and from the blade root to the maximum chord length portion, a crossing angle of an orientation direction of the front surface relative to the direction of rotation gradually decreases, and - the back surface is gradually inclined in a direction of the front surface from the blade root to the maximum chord length portion according to the continuous decrease of the interval between the front and back surfaces (1D, 1E) from the blade root to the maximum chord length portion in the side view.