The study is to propose a novel design for the lower limb rehabilitation system through the motion training of stepping upward or downward. A main goal of this design is to constrain the system dimension to a small value for home use purpose. It is also desired that the functioning of system is automatic that the direction of stepping upward or downward can be changed arbitrarily in the training process. The TRIZ theory, which offers a systematic thinking process for problem solving, is adopted in this study to construct the first step of conceptual design. From the theory, the problems of system design is mainly to deal with three issues, which are the spatial analysis and optimization, motion conversion design for ladder, and the motion control of ladder. The spatial analysis is to investigate the optimal trajectory of ladder motion that the total space of the system can be minimized. It is done by assuming a specific geometrical property of ladder trajectory and solving the optimization problem through the parameter optimization. The motion conversion design is to invent an approach to change the configuration of ladder from either open-to-close or close-to-open status. To do so, a small section of ladder trajectory is asked to be flexible and a mechanism to trigger the configuration change is proposed. The motion control is to maintain the user located in an almost same location while the training is preceded. In this study, the PID control scheme is adopted to examine whether the control goal can be properly achieved. This study reveals that a proper design can be obtained through the specific process of problem solving. The system is shown to function well when the above three issues are solved by the proposed schemes.