Since a groove bottom radius r varies linearly between gradient angles θ1 and θ2 in a tool axis O direction, and the gradient angle θ1 on the tool distal end side is smaller than the gradient angle θ2 on a shank 12 side, it is possible to improve chip discharge performance by reducing the core thickness on the tool distal end side where the diameter dimension of an outer peripheral cutting edge 18 decreases, while maintaining strength and rigidity. That is, since the gradient angle θ1 of the tool distal end side part is small, loss of strength and rigidity resulting from the core thickness becoming extremely small is suppressed, in addition to which, on the shank 12 side having the large gradient angle θ2, the core thickness increases with a large rate of change toward the shank 12 side, and therefore breakage of the like of the tool as a whole can be suppressed by ensuring appropriate strength and rigidity. Further, since the gradient angles θ1, θ2 change only at a change point S, a twisted flute 16 can be machined easily using one flute grinding process, in addition to which the flow of chips in the twisted flute 16 is smoother than if a step is provided, allowing chip discharge performance to be maintained satisfactorily.