A ball end mill includes an end mill body, an indexable cutter insert, a clamp screw and shift preventing mechanism. The body has an insert receiving recess formed in a forward end portion thereof. The insert has a front face, a rear face and at least two convexly curved side faces, and has at least two convexly curved main cutting edges each defined by the intersection of a respective one of the convexly curved side faces with the front face. The insert is received in the insert receiving recess with the rear face mated with the bottom face of the recess in such a manner that one of the main cutting edges is indexed in a cutting position. The shift preventing mechanism includes an engaging recess formed in one of the bottom face of the insert receiving recess and the rear face of the insert and an engaging projection complementary to the engaging recess and provided on the other of the bottom face of the insert receiving recess and the rear face of the insert so as to be brought into engagement with the engaging recess.