It is an object to provide an antiviral agent that can be used for persons having sensitive skin or on the face, inactivates viruses such as a norovirus and an influenza virus, and is excellent in germicidal properties. Further provided is a cleansing agent that does not lead to environmental pollution since the cleansing agent is easily decomposed in the natural environment, scarcely causes eczema and allergic dermatitis since no germicidal agent is added, and has an antiviral performance. The antiviral agent containing a surface-active agent having a C18 unsaturated alkyl group as an active component. It is not always necessary to lather or rinse off with water like cleansing agents such as medicated soaps since the antiviral agent of the present invention at a very low concentration can inactivate the virus.
