Fundamental concepts are presented for realizing hierarchical bottom-up model building in managerial decisions using a model description language with formal semantics. Two types of model representation are introduced. One is a model description, which is a representation of the internal structure of models. The other is an executable module, which is used in model integration context, and is the conceptualization of a model as an input-output functional system. The distinction and relationship between these two types of representation are discussed formally and strictly in the model theoretic way. Finally, a prototype system of a model integration environment will be implemented based on these concepts.