An object of the present invention is to provide a method for improving a proportion of capsids containing a gene of interest GOI in a method for producing a virus, and to provide a method for improving a proportion of capsids containing a gene of interest GOI in a method for introducing a nucleic acid into cells, the method including a nucleic acid introduction step of adding a nucleic acid to a suspension containing cells. According to the present invention, there is provided a method for producing a virus, the method including a nucleic acid introduction step of adding a nucleic acid to a suspension containing cells having a density of 10×106 cells/mL or more, and a culture step of culturing the cells into which the nucleic acid has been introduced, in which the nucleic acid is added in a case where a population cell doubling time of the suspension satisfies 35.4 hours or less.