Environmental problems such as resource depletion and climate change are the most serious problems for society and industry. In order to deal with such problems, scenario writing is a useful methodology for envisioning ideal future, such as sustainable society. Although describing these scenarios requires a lot of time and a large amount of work, there is no computational support for it. In order to resolve this problem, this paper proposes a design methodology of scenarios based on forecasting approach, which explores future from current situation. In order to realize computational scenario design support, we formalize the design process as four steps; setting problems, constructing causal networks, describing storylines, and describing scenario texts. And we develop Scenario Design Support System for supporting the scenario design process on a computer. For testing the proposed methodology, we designed "Electric Vehicle Diffused Society Scenario" as a case study. In the case study, proposed process and design support methodology were useful for designing a forecasting scenario.