|| New industries of the 20th century characteristically involve complex systems. We try to build a concept of ``Learning by Decomposing.'' A product as a system can be designed through the integration and combination of several subsystems, with the interaction between subsystems. What are important in designing a system is how it should be decomposed into subsystems, and how the interface between subsystems should be designed. To find out the best way of decomposing the whole product as a system, abundant knowledge, experience, and know-how would be indispensable. There must be some learning process behind the process of finding out decomposition.