In this paper, we popose a decentralized control algorithm of multiple mobile robots which have been developed for manipulating a large or a long object in cooperation with a human. In this algorithm, each robot is controlled as if it has a caster-like dynamics around a representative point attached to the object and the manipulation of the large or the long object is realized easily in cooperation wit a human. The proposed control algorithm is experimentally applied to two omni directional mobile robots referred to as DR Helper and experimental results illustrate the validity of the proposed control algorithm.