A method for obtaining a virus vector free of a serum, and a serum-free medium for cultivation of a virus producer cell which can be used for the method are provided. By cultivating a virus producer cell using a serum-free medium containing serum albumin, it is possible to cultivate the cell in a state equivalent to that in a serum-containing medium to produce a virus vector at a sufficient titer. The virus vector prepared from the virus producer cell cultivated in the medium exhibits gene transfer efficiency comparable to that of a conventional vector. The medium is useful for gene therapy and studies thereof.