To provide a plasmid vector having high productivity which can be used for large scale production of an industrially valuable and useful protein, and a transformant using the plasmid vector. A plasmid vector having DNA which encodes a plasmid replication protein in which one or more amino acid residues that are selected from (a) position 48, (b) position 262, (c) position 149 and (d) position 198 in the amino acid sequence represented by SEQ ID NO: 2 are substituted with (a) Ala, Gly, Thr, Arg, Glu, Asn or Gln, (b) Gly, Ser, Thr, Cys or Val, (c) Asn, and (d) Glu, respectively.