Function point analysis(FPA) was proposed to help measure the size of a computerized business information system. It is widely used in the actual software development. However, it has been reported that since function point counting involves judgment on the part of the counter, some difference for the same product would be caused even in the same organization.In this paper, we propose detailed FPA measurement rules for the design specifications based on the UML(Unified Modeling Language) and develop the function point measurement tool, whose input products are design specifications on Rational Rose. We have also applied the tool to the actual design specification and examine the difference between the values by the tool and one by the specialist of FPA. The results show the applicability of our tool.