To provide a system of data linkage capable of conversion and registration of substantial same data at ease in spite of inter different systems.

A system of data linkage 20 comprises a first system 21 to be a transfer origin, a data conversion system 23 to convert data transferred from the system 21 and a second system 22 to be a transfer target. Data read from a transfer origin database 26 and transferred from a transfer origin server 25 is converted into data responding to the system 22 by a linkage server 35 based on a conversion table stored into a linkage database 36. The converted data transferred from the server 35 is stored into a transfer target database 32 to be used as like usual data pre-registered in the system 22 by transfer target terminal devices 33a-33c.