A kneading segment, comprising: plural wing portions adjacent to each other along an axis such that angles, around the axis, of the plural wing portions are shifted, wherein each wing portion projects radially outward from the axis and has leading ends at both ends thereof, wherein a base part where leading ends radially project from an adjacent wing portion is a base end of the wing portion, wherein a surface connecting the base end and the leading end and being orthogonal to an axial direction of the kneading segment is a wing surface, a surface corresponding to the outer periphery of the wing portion and existing along said axial direction is a wing side surface, and a surface located at the leading end and existing along said axial direction is a wing leading end surface, characterized in that the plural wing portions are integrally formed, and that at a part of the leading end of each wing portion, part where the wing leading end surface and the wing surface connect with each other, a first flow surface continuing from the wing leading end surface and the wing surface is formed, and wherein at a part of the base end of each wing portion, part where the wing surface and the wing side surface of another wing portion adjacent to each wing portion connect with each other, a second flow surface continuing from the wing surface and the wing side surface is formed, wherein the second flow surface is formed by a concave-curved surface and a radius of curvature of the concave-curved surface formed at the second flow surface is at least 20% of a thickness of the wing portion, or the second flow surface is formed by one or more sloped surfaces and wherein the width of the second flow surface in a direction parallel to said axial direction is at least 20% of the thickness of the wing portion. plural wing portions adjacent to each other along an axis such that angles, around the axis, of the plural wing portions are shifted, wherein each wing portion projects radially outward from the axis and has leading ends at both ends thereof, wherein a base part where leading ends radially project from an adjacent wing portion is a base end of the wing portion, wherein a surface connecting the base end and the leading end and being orthogonal to an axial direction of the kneading segment is a wing surface, a surface corresponding to the outer periphery of the wing portion and existing along said axial direction is a wing side surface, and a surface located at the leading end and existing along said axial direction is a wing leading end surface, characterized in that the plural wing portions are integrally formed, and that at a part of the leading end of each wing portion, part where the wing leading end surface and the wing surface connect with each other, a first flow surface continuing from the wing leading end surface and the wing surface is formed, and wherein at a part of the base end of each wing portion, part where the wing surface and the wing side surface of another wing portion adjacent to each wing portion connect with each other, a second flow surface continuing from the wing surface and the wing side surface is formed, wherein the second flow surface is formed by a concave-curved surface and a radius of curvature of the concave-curved surface formed at the second flow surface is at least 20% of a thickness of the wing portion, or the second flow surface is formed by one or more sloped surfaces and wherein the width of the second flow surface in a direction parallel to said axial direction is at least 20% of the thickness of the wing portion.