In our current research, we are developing a force assistance system for standing up motion. Our developing system realizes the standing up motion using the support bar with two degrees of freedom and the bed system which can move up and down. In this paper, we develop the force assistance system which realizes the natural standing up motion using remaining strength of the patient Our key ideas are two topics. The first topic is distributed system which controls the support bar and the bed system with coordination among them. The second topic is the combination of force and position control. According to the patient's posture during standing up, our control system selects more appropriate control method from them. We use the reference of standing-up motion which is based on the typical standing up motion by nursing specialist for realizing the natural assistance. The performance of our proposed control scheme is experimented by computer simulations.