To improve processing accuracy and roughness of a processed surface achieved by a bottom blade having an R shaped portion.SOLUTION: A ball end mill 1 includes a cutting blade part 3 at a tip side of a tool body 2 rotatable around a center axis O. Two R blades 5 formed on an apical surface of the cutting blade part 3 are disposed facing each other and a gash groove 7 forming a rake surface 8 of the R blade 5 is provided at a front side as seen in a rotation direction of each R blade 5. An outer peripheral blade 13 having a back taper angle is provided at each R blade 5, and a flute groove 16 for discharging chips is provided at the front side as seen in the rotation direction of the outer peripheral blade 13. In the R blade 5, an arc blade 5a formed in a range of an angle θ extends at an intersection part r spaced apart by a quarter arc from a tip part of the cutting blade part 3. The outer peripheral blade 13 is connected to a terminal part s of the arc blade 5a through a back taper connection blade 13d. The gash groove 7 forms a gash surface from the tip part p to an intersecting ridge line part 20 with the flute groove 16. The gash surface forms rake surfaces of the arc blade 5a and the back taper connection blade 13d.SELECTED DRAWING: Figure 4