The objective of inverse manufacturing is to reduce production and consumption volume of artifacts to an adequate and manageable size, while maintaining quality of life and corporate profits. Upgradable artifacts will satisfy this condition, because they will have longer life, encourage reuse of products, and therefore, create new business chances at later stages of product life cycles. This paper proposes a design methodology for upgradable products. First, we clarify the definition of upgradable design by comparing it with other design strategies including traditional design and series design. Second, by analyzing design data of existing products, we propose four design patterns valuable for upgradable design and two basic guiding principles; namely, "make functions independent" and "make functions insensitive". After formalizing these design guiding principles, a result of experimental upgradable design is illustrated. This result shows that upgradable design is effective for reducing total manufacturing costs of a series of products and, therefore, it can be practiced immediately.