Since an MIB file present in a network device and having the structure of MIB data is expressed in the ASN1 format, the MIB file is not flexibly modified by a user.A management-information database-structure definition file converter unit 22 converts MIB files 21a, 21b . . . having the MIB data structures of MIB databases 12a, 12b, . . . mounted on the network devices 10a, 10b, . . . into files in a tag-structured format. Specifically, the management-information database-structure definition file converter unit 22 converts MIB files 21a, 21b, . . . into MIB files 23a, 23b . . . in a tag-structured format (XML format). Using the MIB files 23a, 23b, . . . in the XML format, a network device management tool 24 manages the MIB databases 12a, 12b . . . of the respective network devices 10, 10b . . .