A few decade years have passed since the Object/Relational mapping problem was recognized. However, it still remains to be a hard problem. Simple declaration cannot overcome the impedance mismatch, however, detailed declaration tends to get into a number of duplicated definitions of elements. Duplicated definitions are not preferable in order for productivity and maintainability.In this paper, we propose DBPowder-mdl to address the problem above. DBPowder-mdl describes mappings between object models and relational models. Both object models and relational models are derived from the mapping declaration. DBPowder-mdl keeps simple declaration with having a descriptive power of detailed declaration such as inheritance in object models.We developed a prototype system of DBPowder-mdl and evaluated it.