Environmentally conscious design or ecodesign plays one of the most important roles to create products with less environmental impact targeting the sustainable society. Manufactures often use checklists to support design improvements of products and to obtain eco-labels, such as Eco Mark in Japan. Current checklists are, however, insufficient to support designing products rationally because the relationship between the individual requirements of current checklists and environmental impact is undetermined. This paper proposes a method for supporting ecodesign assessment by developing a weighted checklist from a conventional checklist. This weighted checklist calculates ecodesign achievement based on the potential environmental improvement of each requirement, derived by the life cycle simulation. The result of a case study involving a digital duplicator showed the proposed method successfully clarified requirements that should be improved in the present product. When design improvements are applied, the assessment of the product's CO2 emission is improved by 8%.