Recently, ontology is expected to contribute to sharing and reuse of knowledge, and a lot of research on ontology has been carried out. However, useful methodologies for building ontologies are not yet established in spite of the understanding of its necessity. The crucial point of building ontologies is to articulate concepts in the target field. If a concept is defined in a manner depending on a particular context, it is difficult to reuse it in other contexts. Therefore, it is important to distinguish context-dependent concepts from context-independent ones. The difficulty of the discrimination requires development of a methodology and a guide system for building ontologies. This paper proposes AFM (Activity-First Method), a methodology of building domain ontology depending on task analysis, and an ontology building guide system based on AFM. The guide system supports building ontologies by providing an ontology author with building steps, and managing intermediate results. In the first half of this paper, we take an ontology of oil refinery plant operation task as an example and discuss articulation of the role concepts. Role concepts represent conceptual categories of roles of matters in a particular context. This consideration serves as the foundation of this guide system. In the second half of this paper, we show how the guide system supports an ontology building process.