Abstract: A method of isolating viruses from a liquid with the possibility of containing viruses, including a process of mixing a liquid with the possibility of containing viruses and a water-soluble cationic magnetic fine particle aqueous solution to form conjugates of the cationic magnetic fine particles and the viruses in a mixed solution, a process of mixing a masking agent and an aggregating agent in the mixed solution to form a magnetic composite in which the conjugates are aggregated, and a process of recovering the magnetic composite by magnetic separation, wherein the masking agent is a poly(meth)acrylic acid having a mass-average molecular weight of 5,000 to 100,000, and the poly(meth)acrylic acid is mixed in the mixed solution in a concentration range of 0.01 to 0.1 mass %.