A mobile manipulator consisting of a 6 d.o.f manipulator and an omni-directional moving platform with four wheels is developed. The authors propose a strategy for handling object by cooperation of three omni-directional mobile manipulators using passive joints. Passive joints are free to move joint's movable directions are installed at the tip of each robot to avoid excessive inner force due to mutual positioning errors. Three passive joint mechanisms consist of four passive joints with electro-magnetic brake. The most important thing in this study is simplification of the control using passive joint. The authors propose equation of load on each robot as one of the simplification of complex control with a passive joint.