In a system development, the knowledge of a target business is very important factor for the success of a development. The needed part of such knowledge is different for each stakeholder. Therefore, we need a method to measure the level of understanding of a stakeholder and a method to categorize business knowledge. This paper proposes the metric of difficulty level in order to measure the business knowledge. Moreover, we performed the experiment to confirm the suitability of the proposed metric. Categories and Subject Descriptors D.2.1 [Software Engineering]: Requirements / Specifications; D.2.8 [Software Engineering]: Metrics General Terms Domain knowledge Keywords Domain knowledge, Usecase diagram, Each stakeholder, Difficulty level