Disclosed herein is an antiviral agent against animal viruses. The antiviral agent contains a protein or a nucleic acid sequence encoding the protein, as an active ingredient, the protein having binding ability and degrading ability to foreign nucleic acid chains invaded in an animal cell and that has no cytotoxicity to the animal cell itself. Disclosed herein is further an antiviral animal cell containing the protein according to the present invention, or the nucleic acid sequence encoding the protein. The antiviral agent and antiviral animal cell exhibit advantageous effects in that they selectively degrade foreign nucleic acid chains invaded in an animal cell and have no cytotoxicity to the animal cell, thus causing no death of the animal cell.