A method of producing a recombinant retroviral vector in which a foreign gene has been introduced, comprising the steps of; preparing a retroviral vector comprising a replication origin, sequence A, marker gene X, sequences C and D for introducing a foreign gene by homologous recombination, and sequence B that is homologous with said sequence A, in that order, wherein said sequences C and D are directly or indirectly adjacent, and for use in introducing the foreign gene between said adjacent sequences C and D; preparing a nucleic acid fragment containing sequence C' that is homologous with said sequence C, a foreign gene, and sequence D' that is homologous with said sequence D, in that order; by exposing said retroviral vector and said nucleic acid fragment to conditions to cause homologous recombination, generating a recombinant retroviral vector in which the foreign gene has been introduced between said adjacent sequences C and D, and forming a host that contains the recombinant retroviral vector in which the foreign gene has been introduced between said adjacent sequences C and D; culturing the host obtained in Step, and selecting, from the culture, a host containing the recombinant retroviral vector in which the foreign gene has been introduced between said adjacent sequences C and D by identifying the expression of marker gene X as an indicator; and extracting from the host selected in Step the recombinant retroviral vector in which the foreign gene has been introduced between said adjacent sequences C and D, wherein said sequence A is 3 'LTR contained in a retroviral genome and said sequence B is 5' LTR contained in a retroviral genome, or said sequence A is 5' LTR contained in a retroviral genome and said sequence B is 3' LTR contained in a retroviral genome. preparing a retroviral vector comprising a replication origin, sequence A, marker gene X, sequences C and D for introducing a foreign gene by homologous recombination, and sequence B that is homologous with said sequence A, in that order, wherein said sequences C and D are directly or indirectly adjacent, and for use in introducing the foreign gene between said adjacent sequences C and D; preparing a nucleic acid fragment containing sequence C' that is homologous with said sequence C, a foreign gene, and sequence D' that is homologous with said sequence D, in that order; by exposing said retroviral vector and said nucleic acid fragment to conditions to cause homologous recombination, generating a recombinant retroviral vector in which the foreign gene has been introduced between said adjacent sequences C and D, and forming a host that contains the recombinant retroviral vector in which the foreign gene has been introduced between said adjacent sequences C and D; culturing the host obtained in Step, and selecting, from the culture, a host containing the recombinant retroviral vector in which the foreign gene has been introduced between said adjacent sequences C and D by identifying the expression of marker gene X as an indicator; and extracting from the host selected in Step the recombinant retroviral vector in which the foreign gene has been introduced between said adjacent sequences C and D,