The antiviral agent according to the present invention comprises an inorganic solid acid that has an acid site concentration exceeding 0.005 mmol/g. The inorganic solid acid preferably comprises an inorganic phosphoric acid compound, an inorganic silicic acid compound or an inorganic oxide. In the inorganic solid acid, the acid strength (pKa) of acid sites is preferably not higher than 3.3.本發明之抗病毒劑係包含酸基(acid site)濃度超過0.005mmol/g的無機固體酸。無機固體酸較佳包含無機磷酸化合物、無機矽酸化合物或無機氧化物。無機固體酸之酸基的酸強度(pKa)較佳為3.3以下。