To generate a large rotational force to a main shaft without increasing the radius of rotation of an arm.

A joint device fitted to a tip of an arm 1 to be rotated by a main shaft turns the arm by a sprocket and a gear in a reverse direction to that of the main shaft to keep an arm 2 constantly in an approximately horizontal condition. The force applied to a tip of the arm is applied to a tip of the arm 1 by an eccentric shaft fixed to a side of a turning gear, and a large rotational force is generated on the main shaft when the arm 1 and the arm 2 are horizontal and the angle formed therebetween is 180°.