This paper proposes a simple method for applying popular PID position-controlled industrial robots to cooperative tasks with force interactions among them. For handling single object by multiple robots, force control using torque controllers is generally used to avoid excessive inner forces, however, a popular industrial robot has a PID positioncontroller. Thus it is usually very difficult to realize cooperative control with industrial robots. This paper proposes a method to introduce flexible mechanisms at the tips of robots to control forces at the end-effectors. After the installation of flexible mechanisms at the tips of robots, a closed-loop structure including flexible parts is generated. First, the number of degrees of freedom of the closed-loop structure is analized by modeling the installed flexibility as virtual joint variables. Next, the minimum number of springs, required to avoid both excessive inner forces and vibrations caused by the installed flexibility, are derived. Finally, the minimum number of actuators to control the object in arbitrary directions of 3 D space is derived for installed robots.