Provided in the present invention is a novel use of composition containing a neoagarooligosaccharide mixture as an active ingredient. The neoagarooligosaccharide mixture and the like as an active ingredient of the composition according to the present invention can reduce the expression of RANKL, an osteoclast inducer, or IL-6, an inflammatory cytokine, and effectively inhibit differentiation of monocytes into osteoclasts. Thus, the composition according to the present invention may be used as a medicine or functional food for preventing, alleviating, or treating arthritis (especially rheumatoid arthritis). Further, the active ingredient of the composition according to the present invention can be obtained by an enzyme reaction between a readily available substrate such as agar or agarose and DagA as a beta-agarase derived from Streptomyces coelicolor. Thus, it can be produced at a relatively low cost and can easily be ingested by anyone because it has no side effects.