Descriptions of program identifiers improve the maintainability of programs. Modern software projects maintain proper descriptions by following coding conventions. However, software projects maintained for a long time have two problems: (i) descriptions at incorrect locations and (ii) no descriptions. We propose the method of generating a identifier dictionary for managing identifiers and their descriptions, which enables developers to refer to identifier descriptions from anywhere within programs. The method involves two steps: (i) extracting identifiers and descriptions from design documents and programs and (ii) generating descriptions using information-retrieval and machine-learning methods. We applied the proposed method to COBOL programs and design documents of a legacy system that has been maintained for over 20 years as a case study. The proposed method obtained the descriptions of 83% of identifiers and reduced the cost of locating files to be modified by enhancing search keywords using the identifier dictionary. This means that the proposed method can improve the maintainability of systems maintained over many years.