1. A computer system comprising: one or more application servers on which an application program that issues a database processing request conforming to a first database operation language in accordance with a database independent protocol runs; an old database server which receives the database processing request conforming to the first database operation language in accordance with a database dependant protocol; a new database server which receives a database processing request conforming to a second database operation language in accordance with the database dependant protocol; and a database connecting unit which converts the database processing request, which conforms to the first database operation language and is issued from said application program, from its protocol into the database dependant protocol suitable for said old database server to provide the converted database processing request to said old database server, and converts said database processing request, which is not converted from the first database operation language into the second database operation language, from its protocol into the database dependant protocol suitable for said new database server to provide the database processing request to said new database server, converts data processing results of said old database server and said new database server from their protocol into the database independent protocol and compares and verifies the data processing results to output the comparison and verification result, and returns the data processing result of said old database server converted from its protocol into the database independent protocol to said application program when a preparation mode is designated, and converts the database processing request issued from said application program from the first database operation language into the second database operation language in accordance with a conversion rule which is set based on the comparison and verification result, and then converts the database processing request from its protocol into the database dependant protocol suitable for said new database server to provide the database processing request to said new database server, and converts the data processing result of said new database server from its protocol into the database independent protocol to return the data processing result to said application program when a migration mode in which said old database server is completely migrated to said new database server is designated.