Continuing optimal product line development needs to evolve core assets in response to market, technology or organization changes. In this paper, we propose a product line evolution method based on the kaizen approach. Kaizen is a continuous improvement method that is adopted in Japanese industry. The important points of the kaizen are to prepare a work standard and continue to improve processes by correcting the differences between the standard and actual results. Our core asset kaizen method provides a standard that includes core asset types based on simple metrics, kaizen patterns representing expertise, and kaizen processes for continuous improvement.