An anti-avian influenza virus agent containing, as an active ingredient, a component obtained when a guava extract extracted from a guava with an extraction solvent consisting primarily of at least one of water and a hydrophilic organic solvent is subjected to fractionation by HPLC under specific conditions. The specific conditions include the use of an ODS column, the use of a water-methanol mixture having a methanol concentration of 20% as a mobile phase during a period until 15 minutes after applying the guava extract to the column, and the use of a water-methanol mixture having a methanol concentration of 40% as a mobile phase during a subsequent period of 15 to 30 minutes. Alternatively, the specific conditions further include the use of a water-methanol mixture having a methanol concentration of 60% as a mobile phase during a period of 30 to 45 minutes after applying the guava extract to the column. The component contained in the anti-avian influenza virus agent is eluted from the column in a period until 20 minutes after applying the guava extract to the column or in a subsequent period of 20 to 35 minutes.
