A standing-up assistance chair without any electric actuator for the weak-muscle people, such as elderly people and patients, was developed in this research. The mechanism was designed based on the real human standing-up motion. To satisfy the motion, the design principles were considered, 1) the timing of opportunity of leaning forward motion (forward movement of shoulder) before lifting the hip joint is 40% of standing-up phase; 2) The trajectory of user’s hip joint is a straight line with angle of 45 [deg]; 3) the device supports the user until his/her knee joint angle achieve around 60 [deg]. The mechanism with eight linkages based on Hart’s exact straight-line mechanism was considered, which can realize the motion of the approximate straight line with an angle of 45 [deg]. Based on the real human motion and design principles, the proper link lengths of the device were simulated to observe the effect of hip joint angle and the trajectory of COG. We confirmed that during the phase of standing leaning forward and recovered motion can be assisted with an adequate method. By using our assistance chair, the torque on each joint can be decreased. For different sizes of users, we intend to build S, M, L types of the device. Based on the average human body data .