A rotary blade for a weed cutter, in which a center face part including an axial center of a thin disc body 1 wholly made of a spiral steel material is in the form of a horizontal plane, and a circumferential part adjacent to the center face part has a one rotation-directional corrugated cutting part having one side face of concave form and the other side face of convex form, wherein the cutting part face is corrugated in the circular curved-surface form instead of the triangular form.
