A composite sintered body cutting tool made of a composite sintered body of: a TiCN-based cermet layer; and a WC-based cemented carbide layer, wherein (a) an angle between a rake face and a flank face of the cutting tool is 90°, (b) the rake face including a cutting edge of the cutting tool is constituted from the WC-based cemented carbide layer, in which 4 to 17 mass% of an iron group metal component and 75 mass% or more of W are included; and a major hard phase component is WC, (c) the thickness of the WC-based cemented carbide layer is 0.05 to 0.3 times of the thickness of the composite sintered body, (d) the TiCN-based cermet layer is constituted from a single layer of a TiCN-based cermet layer, comprising at least 4 to 25 mass% of iron group metal components comprising Co and Ni, less than 15 mass% of W, 2 to 15 mass% of Mo, 2 to 10 mass% of Nb and 0.2 to 2 mass% of Cr in a case where contents of constituting components of the cermet layer are expressed as contents of metal components, and satisfies a Co content relative to a total content of Co and Ni of 0.5 to 0.8 with respect to Co and Ni of the iron group metal component in a mass ratio, and (e) when a height profile from an upper end to a lower end of the flank face is measured in a plane, which passes a center of the rake face of the cutting tool and is perpendicular to both of the rake face and the flank face, as a line, which passes a ridge line where the rake face and the flank face intersect and perpendicular to the rake face, being a reference line, a maximum elevation difference value of the height profile is in a ratio of 0.01 or less with respect to a thickness of the composite sintered body from a front surface of the rake face to a rear surface. (a) an angle between a rake face and a flank face of the cutting tool is 90°, (b) the rake face including a cutting edge of the cutting tool is constituted from the WC-based cemented carbide layer, in which 4 to 17 mass% of an iron group metal component and 75 mass% or more of W are included; and a major hard phase component is WC, (c) the thickness of the WC-based cemented carbide layer is 0.05 to 0.3 times of the thickness of the composite sintered body, (d) the TiCN-based cermet layer is constituted from a single layer of a TiCN-based cermet layer, comprising at least 4 to 25 mass% of iron group metal components comprising Co and Ni, less than 15 mass% of W, 2 to 15 mass% of Mo, 2 to 10 mass% of Nb and 0.2 to 2 mass% of Cr in a case where contents of constituting components of the cermet layer are expressed as contents of metal components, and satisfies a Co content relative to a total content of Co and Ni of 0.5 to 0.8 with respect to Co and Ni of the iron group metal component in a mass ratio, and (e) when a height profile from an upper end to a lower end of the flank face is measured in a plane, which passes a center of the rake face of the cutting tool and is perpendicular to both of the rake face and the flank face, as a line, which passes a ridge line where the rake face and the flank face intersect and perpendicular to the rake face, being a reference line, a maximum elevation difference value of the height profile is in a ratio of 0.01 or less with respect to a thickness of the composite sintered body from a front surface of the rake face to a rear surface.