It is indispensable for strategic product line development to define the proper scope of the product line. Once the scope has been defined, we examine the corresponding product line architecture to realize systematic reuse for the product line. Therefore, in defining the scope, we have to decide whether or not it is appropriate to share the same architecture for the products in the product line. The appropriateness of sharing the same architecture among multiple products has to be examined from two points of view. One is from the point of view of the individual optimality (i.e., whether it is good for each product to use the shared architecture), and the other is from the point of view of the whole optimality (i.e., whether it is good for the product line as a whole to share the architecture). In this paper, we propose a method for product line scoping. We consider scoping as a decision-making activity in which we evaluate multiple candidates for the scope and then select the proper one after examining the appropriateness from the two points of view. In order to demonstrate its applicability, we applied the method to the actual problem picked up from Japanese ITS (Intelligent Transport Systems) projects.