Decubitus ulcer is a disease that causes the necrosis of skin tissue through the disorder of blood circulation on a certain part of the body where bones protrude. This study is aimed at obtaining an optimal design of a set composed of a rubber air-cell seat cushion and back-supporter, showing even distribution of the interface pressure and minimizing its peak value on the buttocks through optimization using the simulation. The finite element (FE) human model was a human body model developed by the MADYMO (mathematical dynamic model; TASS, Netherlands) and consisted of 44 FE models such as skeletal structures, flesh, muscles, and internal organs. For the optimization of a seat cushion and a back-supporter, a basic model for seat cushion and back-supporter were designed using 50th percentile European male model. To design an optimal seat cushion and back-supporter system, the optimization was performed considering the design factors such as the air-cell height of buttocks and thighs, air-cell height of back-supporter, and angle of back-supporter. The results of the sensitivity analysis confirmed the two design factors (height of buttock air-cell and height of thigh air-cell). The optimization result was 55 mm for the buttock air-cell's height, 60 mm for the thigh air-cell's height, 90 mm for the back-supporter air-cell's height, and 100 ° for the back-supporter's angle. Based on this study, there is preliminary evidence to show that the design of seat cushion and back-supporter is necessary to reflect biomechanical characteristics, such as body shape (difference of body shape between thigh and buttocks). A main advantage of air-cell type for seat cushion and back-supporter is the good effect of interface pressure distribution.