Many types of management system audit are widely spread in the companies, e.g., quality management system audit, etc. The management system audit can be regarded as management decision-making. But there are very few decision support systems for the management system audit, since management system audits are different from usual management decision-making. For management system audit management standard is developed, and auditors must verify that an individual management system of a company consistent to the requirements of management system standards. Ontology is information structure, which helps to acquire knowledge, share it, and check consistency within the knowledge. One of our main aims of this paper is to present a methodology of ontologydriven decision support systems for management system audit. Firstly, we characterize the management system audit as a new decision-making. Next, we introduce a concept of ontology formally, and develop generic management system ontology, and company quality management system ontology. Finally we present a methodology of ontologydriven decision support system for management system audit, and show the characteristics of the decision support system.