The topology optimization made a great success in pure structural design in an actual industrial field. However, a lot of factors interact each other in a actual engineering field in highly complicated manner. The typical conceptual trade-off is that cost and performance, that is, since they are competing factors, one can't improve the specific system without consideration of interaction. The vehicle has lots of competing factors, especially like fuel economy and acceleration performance, mass and stiffness, roominess and cost, short front overhang and crash-worthiness and so on. In addition, they interact each other in a more complicated manner, that is, fuel economy has something to do with not only engine performance but also mass, roominess, stiffness, the length of overhang, trunk volume, etc. So, most of decision-makings have been made by management based on subjective knowledge and experience. Especially, since structural and ergonomic parts have been designed separately, a good harmony has not been achieved between two. It is time to expand optimization technique to multi-disciplinary optimization to come up with more accurate decision-making. In this dissertation, the whole optimization model is come up with for higher stiffness, less mass or cost, better styling and ergonomic comfort based on decomposition.