Antiviral agent and antiviral food

Provided are an antiviral agent and an antiviral food as novel methods for utilizing a euglena-derived substance. The antiviral agent and antiviral food, both comprising a euglena-derived substance as an active ingredient, are to be used for preventing or treating an infectious disease caused by an envelope-free RNA virus. Examples of the envelope-free RNA virus include rotaviruses belonging to Reoviridae. Examples of the euglena-derived substance include euglena, a hot water extract of euglena, paramylon and an alkali-treated paramylon.