Provided are a DNA fragment containing a gene relating the autonomous replication of a plasmid, and a DNA fragment containing a gene relating to the autonomous replication of a plasmid wherein a mutation point capable of increasing the number of copies of a plasmid is present in at least one site in the gene. Also provided are a DNA fragment containing a gene having a function relating to the autonomous replication of a plasmid within the bacteria of the genus Rhodococcus, a DNA fragment wherein a mutation point capable of increasing the number of copies of a plasmid is present in at least one site in the gene, and a plasmid carrying a DNA fragment wherein a mutation point capable of increasing the number of copies of a plasmid is present in at least one site of the gene.