Recently, motorized equipment is required in order to support ADL (Activities of Daily Life) of the elderly. Authors are developing mobile type walking support systems. In this development, we understood the requirement for the assistance in standing-up and sitting-down motion is very important. We started to study about the elemental technologies of standing-up and sitting down motion assisted by upper body support arm (which is used for mobile type walking support systems). This paper describes the study on the support arm trajectory during this assisted motion. We developed a program to analyze the load acting lower limb joints in the motion. Using experimental data of supporting force, joint positions, and ground reaction force, joint torque was calculated in some trajectory patterns. Means to evaluate performance of the arm trajectory were obtained.