We address a decentralized control method for object transportation in coordination by a leader-follower type multiple robot system. The proposed system consists of a pushing leader, a human being or a robot without grasping mechanisms, and multiple follower robots. During the object transportation, only the leader has desired trajectory, and follower robots estimate the trajectory of the leader based on force/moment from the object. In the proposed system with the pushing leader robot, imitating the system with human leader, a variable internal force is introduced to each robot's controller in decentralized style to let the follower's estimator work on not only the pushing but also the pulling case that the leader needs to slow down or stop the object. Finally, some experiment results are shown to illustrate the concept of the proposed algorithms.