To provide a taper ball end mill for machining a taper groove to improve surface roughness of a cut surface and a tool life, in the taper ball end mill for machining a taper groove used in machining of the rib groove of a mold.

In the taper ball end mill for machining a taper groove which is used in machining of the rib groove of a mold and where a tip blade is a ball blade and an outer peripheral blade is a taper blade, the number of the outer peripheral blades is set to the even number being four or more blades, two tip ball blades are provided in succession to a pair of outer peripheral blades situated approximately in a symmetrical position as seen in the front view of the end mill, and the position of the tip part of other outer peripheral blade is set to a position, where 0.05-0.5 mm is added to the radius of the tip ball blade, from the tip part of the end mill in the axial direction of the tool. Further, the web thickness of a round 45° part of the tip ball blade is set to 20-50% of the blade diameter of the portion.