First, the development of 'thesaurus' from medieval to present is briefly surveyed with special reference to Roget's thesaurus. The intention of Roget's Thesaurus is to categorize the ideas which are expressible by language and then to formulate English lexical system. It is also asserted that Roget owed his idea of thesaurus to F. Bacon, J.A. Comenius and J. Wilkins. Finally, we discuss what kind of modification Roget's Thesaurus has been receiving to meet the practical demands of the times, and thus examine several types of thesaurus in order to investigate the possiblities of further developments of thesaurus.