PURPOSE: To improve strength and to reduce space by facilitating connection of both outer and connecting flanges in a connecting device for a universal joint, and by reducing the neck length of the connected flanges.

CONSTITUTION: A male screw 23 is formed on one of outer circumferences of an outer flange 21 of a universal joint 22 attached to ends of propeller shafts 1 and 1' and a connecting flange 22 connected to it, and an adapter 25 on a female screw 27 to be screwed with the male screw 23 is formed is fitted to the flange 21. In order to connect the outer flange 21 to the connecting flange 22, the adapter 25 is fitted in the connecting flange 22, this is attached to the end of the propeller shaft 1' and then, opposing faces of the connecting flange 22 and the outer flange 21 are brought into contact with each other, and the adapter 25 is rotated by a rotating tool or the like. Then, the female screw 27 of the adapter 25 is screwed with the male screw 23 and both the flanges 21 and 22 are closely connected to each other.