A persistently infective virus vector is produced by using a gene so modified as to encode an amino acid sequence including a valine substituted for an amino acid residue at position-1618 in the amino acid sequence for an L protein of a persistently non-infective Sendai virus. A non-transmissible, persistently infective virus vector is also produced by defecting or deleting at least one of M gene, F gene, and HN gene. These virus vectors have no cytotoxicity, can achieve the sustained gene expression over a long period of time, is safe, and is therefore useful.