An antiviral agent according to the present disclosure includes, as an active ingredient, an extract extracted from a microalga Pseudochoricystis ellipsoidea Sekiguchi et Kurano gen. et sp. nov. MBIC11204 strain. The microalga may be cultured in a culture medium comprising sufficient nitrogen or may be cultured in a nitrogen-deficient medium thereafter. The extract is extracted from the microalga with alcohol, hot water, or the like as an extraction solvent. The extract may be extracted from the residue thereof with hot water. The antiviral agent may include the extract obtained using one extraction solvent, or may include a mixture of the extracts obtained using multiple extraction solvents.
