To solve the problem wherein grinding a sigmoid end gash requires a skill, because the end cutting edge at the nose of a ball end mill is ball-shaped and is sigmoid as viewed from its end, and various sigmoid shapes subtly vary.

A grinding device like this ball end mill grinds the ball-shaped end cutting edge by turning the ball end mill on a device like a link or the like that works with respect to rotating a headstock plate that supports a rotary grinding wheel. In addition, it has an adjustable speed change device on a device that works with respect to the aforementioned device to cope with various sigmoid end gash shapes by changing the rotations of the ball end mill.