Abstract: It is the object of the invention to provide a chemical substance which is effective, in particular, against insect-transmitted viruses, compared with conventional plant-virus disease preventive agents. This substance is produced from tissues of plants of the genus Mirabilis, and may be widely utilized as a basic protein with anti-plant-virus effects and as an active ingredient for plant-virus diseases preventive agents. Such preventive agents can eliminate difficulties in safety, and is effective because antiviral activity is developed systemically.