During a software system development, existing similar systems are useful because such systems and their documents help developers to understand and reuse problems and solutions of the new system. Especially, such helps are effective if developers are not familiar with the new system. In this paper, we present a supporting tool called "So/M" for an analyst to define software requirements. By using So/M, the analyst can easily refer functions of existing similar systems, their commonality and the relationships among them. So/M also enables the analyst to choose the candidate of requirements, and to generate the skeleton of requirements. During a comparative experiment, we have confirmed that So/M significantly helped analysts unfamiliar with a system to be developed.