Provided in the present invention is a composition having a novel use and containing a neoagarooligosaccharide mixture as an active ingredient. According to the present invention, a neoagarooligosaccharide mixture, which is the active ingredient of the composition according to the present invention, and the like can reduce the expression of RANKL, which is an osteoclast inducer, and effectively inhibit the differentiation of mononuclear cells into osteoclasts. Therefore, the composition according to the present invention can be used as a medicine or functional food for preventing, alleviating, or treating arthritis (especially rheumatoid arthritis) or osteoporosis.
