In order to make decisions for a new system investment, it is important for a decision-maker to forecast what benefits can be achieved after investment. However, it is very difficult to predict the benefits because a new system affects several factors, such as business style, product cost and marketability. To forecast the benefits of a new system clearly, we propose a scenario generation method based on the qualitative simulation with a structural model that derives the scenario documents of new system benefits by determination of an effective functional item. As there are many system functions in the new system, an effective function item is the most suitable system function to acquire the benefits of a desired item. The scenario document is generated when the selected effective function item and the desired item are put into a sentence template. This scenario generation method is applied to the business to business e-commerce system. By using the generated scenario document, the system engineer can know the key points so he can explain the desired benefits of new system to the company which is willing to introduce a new system.