A virtual network system comprising: a version database configured to store against a list of currently used virtual network functions, VNFs, a list of activatable/changeable VNFs that are compatible with the currently used VNFs, and a list of unactivatable/unchangeable VNFs that are not compatible with the currently used VNFs, each of the lists including version information of the respective VNFs; a virtual network function database configured to store, for a plurality of virtual network functions, VNFs, virtual network function data and version information; and a control apparatus configured to: receive, from an operator terminal, an instruction to activate or change a virtual network function, wherein the instruction includes version information of the virtual network function instructed to be activated or changed, and; confirm that the version information included in the instruction is the version information of an activatable/changeable VNFs stored in the version database; and when it is confirmed that the version information included in the instruction is the version information of the information of the activatable/changeable VNFs stored in the version database, acquire the activatable/changeable VNF from the VNF database, set the acquired VNF in the controller of a target physical server, and notify the operator terminal that the acquired VNF has been set; when it is confirmed that the version information included in the instructions is the version information of the inactivatable/unchangeable VNFs stored in the version database, sending to the operator terminal an error message of the activation or change. a version database configured to store against a list of currently used virtual network functions, VNFs, a list of activatable/changeable VNFs that are compatible with the currently used VNFs, and a list of unactivatable/unchangeable VNFs that are not compatible with the currently used VNFs, each of the lists including version information of the respective VNFs; a virtual network function database configured to store, for a plurality of virtual network functions, VNFs, virtual network function data and version information; and a control apparatus configured to: receive, from an operator terminal, an instruction to activate or change a virtual network function, wherein the instruction includes version information of the virtual network function instructed to be activated or changed, and; confirm that the version information included in the instruction is the version information of an activatable/changeable VNFs stored in the version database; and when it is confirmed that the version information included in the instruction is the version information of the information of the activatable/changeable VNFs stored in the version database, acquire the activatable/changeable VNF from the VNF database, set the acquired VNF in the controller of a target physical server, and notify the operator terminal that the acquired VNF has been set; receive, from an operator terminal, an instruction to activate or change a virtual network function, wherein the instruction includes version information of the virtual network function instructed to be activated or changed, and; confirm that the version information included in the instruction is the version information of an activatable/changeable VNFs stored in the version database; and when it is confirmed that the version information included in the instruction is the version information of the information of the activatable/changeable VNFs stored in the version database, acquire the activatable/changeable VNF from the VNF database, set the acquired VNF in the controller of a target physical server, and notify the operator terminal that the acquired VNF has been set; when it is confirmed that the version information included in the instructions is the version information of the inactivatable/unchangeable VNFs stored in the version database, sending to the operator terminal an error message of the activation or change.