The shell of this helical broach (1) is formed by stacking a plurality of wafer shells (20.sub.W(1)-20.sub.W(N)) in the axial direction, and is obtained by forming on the wafer shells (20.sub.W(1)-20.sub.W(N)) finishing blades (30.sub.W(1)-30.sub.W(N)) corresponding to teeth grooves on a piece to be cut (W) and forming the finishing blades (30.sub.W(1)-30.sub.W(N)) such that the blade width gradually increases with each of the aforementioned wafer shells (20.sub.W(1)-20.sub.W(N)) from the leading end of the cutting direction toward the trailing end of the cutting direction.