The purpose is to improve the surface roughness Rz of a cut surface to 1.6 mu m or under in cutting e.g. hardened steel. The ball end mill has a cutting blade made of a polycrystalline hard sintered material containing 20 to 95 vol% of cubic boron nitride. The cutting blade has a ridge portion having a radius of curvature of 5 to 30 mu m. A flank and a rake face or a negative land of the cutting blade connect smoothly with the ridge with the radius of curvature. The ridge portion of the cutting blade has a surface roughness of 0.1 to 1.0 mu m in terms of 10-point average roughness (Rz). <IMAGE>
