The present invention provides a shuttle vector for amicroorganism of the genus Bifidobacterium (BM) and Escherichiacoli having a wide host range and a large copy number in BMand capable of highly expressing a desired protein when usedas an expression vector； an expression vector capable ofexpressing a desired gene in BM by use of the shuttle vector；BM transformed with the expression vector； and an antitumoragent comprising the BM as an active ingredient . It comprisesa pTB6-derived region portion comprising a replication origin(oriV) -repB region of pTB6 but not comprising MembB, MobA, OrfI ,and oriT regions of pTB6 and an Escherichia coli-derived plasmidportion comprising a replication origin (oriC) region ofEscherichia coli but having deleted DNA encoding an N- terminalregion of an ampicillin resistance gene (ampR) expressionproduct .beta.-lactamase, and constitute a shuttle vector forEscherichia coli with a BM having an average copy number of6 to 30.
