In our current research, we are developing a power 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 control scheme which realizes the natural standing up motion with fewer loads to the patient. For developing control scheme, we investigate the standing-up motion of aged people who requires to power support and typical standing up motion by nursing specialist. Comparing with two motions, we set the reference of standing-up motion with our system and we discuss the required assistance force during standing up. Our key ideas are two topics. One topic is control reference using zero moment point. Zero moment point is useful index which shows stability of the patient's posture. The other topic is combination of force and position control. According to the patient's posture during standing up, our control system select more appropriate control method from them. The performance of our proposed control scheme is experimented by computer simulations.