PURPOSE: To provide durability for a blade to offer it a long and stable service life by forming grooves along opposed connecting faces of split halves of the blade, disposing engagement pieces which are engageable with each other in the grooves and integrally joining them.

CONSTITUTION: Halves of a blade which are indicated by 21 and 22 have connecting faces 31 and 32 along which grooves 23 and 24 are cut out thereon, respectively. Opposed ends of the halves at the grooves are made substantially equal to the original biades 21, 22 in thickness, on account of which the configuration of the grooves is protruberant outward to some extent on the blade face and exhibits Y-shape in section. Engagement piece 33 is engaged with both of said grooves 23 and 24 and a binder 34 interposes both between the engagement piece 33 and groove 23 and between said piece 33 and groove 24. The binder 34 is adapted for tight bonding of the blade halves 21 and 22 and for resisting against the air rushing through the connection. Since the connection may be prevented from being broken or raptured, the blade can be stably used for a long period.