PURPOSE: To facilitate the adjustment of the number of natural vibration of a blade by a method wherein a reinforcing rib is provided at the root of the blade and the rigidity or the moment of inertia of the blade is changed to adjust the number of natural vibration of the blade.

CONSTITUTION: A blade 11 is made of resin or the like and the number of natural vibration of the blade is adjusted by increasing the rigidity of the blade by a method wherein the reinforcing rib 13, made of resin or the like, is laminated to cover the external surface of the root 12 of the blade. The number of natural vibration of the blade is changed by changing the height of the reinforcing rib. The number of natural vibration of the blade may be adjusted arbitrarily by the material of the blade, the covering area of the reinforcing rib and the position of the reinforcing rib at the root of the blade.