The present invention provides an antiviral agent comprising an RNA oligonucleotide having a particular sequence and structure. Specifically, when a cell line is treated with an RNA oligonucleotide having specific sequence and helical bend structure according to the present invention, the expression of interferon-β or ISG56 is increased and anti-viral activating is exhibited. Thus, a composition comprising the RNA oligonucleotide can be used as an antiviral agent