It is a difficult task to construct a product model that is a basis and a requisite for managing information about products totally and effectively through their life cycle. ISO 10303 or STEP that is an international standard for the computer-interpretable representation and exchange of product data takes a two-step approach that is, first to model application requirements and generic representation separately and then to make mapping between these two models. However, reviewing Application Protocol standards of STEP, we have found issues probably due to the insufficient mapping methodology. To remove deficits caused by the complications of mapping, we have developed a software tool that helps users who make mapping with graphics. The tool is developed based on mapping model that represents detailed mapping relation between models. In STEP, constraints can be added to the model using EXPRESS formal description language to make the two models consistent. We also have found the basic nature of the language disturb the correct description and understanding of constraints. To solve this problem, we propose a language dedicated for the correct mapping of cardinality that generates constraint description in EXPRESS. With this language, cardinality conditions and aggregate information can be described separately and explicitly.