In Japanese and Korean, the kinship terms elder brother and elder sister were originally used for addressing family members. However, these kinship terms can also be used to address people who are not family members. This usage is called the fictive use of kinship terms. In a questionnaire, the present study asked whether a person addresses others at work as elder brother and elder sister in Japan and Korea. A total of 324 office workers, 148 Japanese from the Hiroshima area in Japan and 176 Koreans from the Seoul area in Korea are participated in the study. The analyses of the study showed the following interesting results. First, in an office situation, Koreans were likely to use the kinship terms more frequently than Japanese. Koreans tend to build up a closer relationship with coworkers by frequently using the kinship terms elder brother and elder sister. In contrast, the Japanese seldom address people at work by these kinship terms. Second, closeness or intimateness between a speaker and listeners at work was a major factor in the use of kinship terms. Both Japanese and Korean are likely to use the kinship terms with coworkers when they have a close relationship. Third, these kinship terms are used among people of the same gender. Fourth, working experience at the same company or public office determines frequency of the use, especially among Koreans. The kinship terms are used with persons who entered a company or public office earlier than the speaker, but not with those who entered the same year as the speaker, and to the much lesser degree with those who entered after the speaker. Contrary to Koreans, Japanese seldom use these kinship terms with anybody at work.