This paper deals with a solid model from which form features can be extracted easily in a general framework. The introduction of form feature concept to the solid modeling system has been proposed and proved to be effective in the manufacturing processes such as process planning. However, there are several points left to be improved. This comes from the fact that properties of a form feature are mainly dependent of its input and that they do not have sufficient information to represent another aspect of the objects explicitly. The approach of feature extraction from a solid model is considered as a candidate to overcome the problems. Some studies regarding to the form feature extraction has been examined but they are not fully successful. This paper treats the form feature as a set of faces with partially common properties. In the proposed model, properties of geometric elements with the same attribute values, such as planes of two parallel faces of a solid, are expressed directly and explicitly, which can be utilized effectively in the extraction. The modeling method is based on the object oriented concept with the extensions of inheritance of instance objects and unification of instance objects.