PURPOSE: To facilitate assembling of a ball joint mechanism by making a shaft- side ball-engaging recessed portion deep enough to accommodate more than half of a ball, and fitting over the shaft a ring-shaped stopper which is energized toward the recessed portion by a spring and whose edge can prevent the ball inserted into the recessed portion from dropping, and allowing the ring-shaped stopper to be freely separated as an outer sleeve is fitted thereover.

CONSTITUTION: A ball-locking recessed portion (a) formed in that portion of an output shaft 3a which is near a transmission case 3 is sufficiently deep to lock more than half of a transmission ball 16a therein. A coil spring 17 and a ring stopper 18 energized by the coil spring 17 toward the recessed portion (a) are both fitted over the output shaft 3a, and the transmission ball 16a inserted into the recessed portion (a) is hooked at the edge of the ring-shaped stopper 18 and so can be prevented from dropping. The outer sleeve 16b of a ball joint mechanism J3 secured to a transmission shaft 10 is inserted from the direction of the axis of the mechanism, and the ring-shaped stopper 18 is thereby pressed in the direction of the axis and unlocks the transmission ball 16a.