In this paper, we proposed one of the methods for building thesaurus in Science & Technology domain and investigated its applicability as an inference service based on ontology. There exist as many building methods for thesaurus as its role and function, and actually many thesauri capable of ensuring the accuracy and efficiency in information search are being built by many experts. After examining the previous studies related to the principles of building thesaurus and relevant concept "facet", we focused on its characteristics and applied it to building thesaurus. The facet is classified into 2 categories, conceptual facet and relational facet. The latter contains 3 subcategories: category relational facet, attribute relational facet and thematic relational facet. The thesaurus for Science & Technology domain using facets can be applied as a web-based inference service. As a result, the three types of inference service, COP(Communities of Practice), Researcher Tracing and Research Map are provided by means of ontology, and can be applied for the Query Expansion.