In this paper, we propose a method of cooperative transportation by two autonomous 4-legged robots. Since a legged robot's body vibrates seriously in walking, it is difficult to apply the same control method that are effective in cooperation of wheeled robots to legged robots. Then we made a flexible joint with elastic rubber to fix an object to the robot. It can sense force between the object and the robot itself. The two robots compose a leader-follower system and each robot controls its walking velocity and walking phase by its own information of force. We show experimental results of cooperative transportation based on the above method.