PURPOSE: To provide an EDI format conversion method by which the item layout of a data base peculiar to a user can freely be changed only by changing the condition setting of a projection processing.

CONSTITUTION: Data derivation parts 12 and 25 deriving data peculiar to the user, which is read from the data base peculiar to the user 11, or received EDI standard data on RDB pseudo tables 13 and 26 in table structure, data extraction parts 14 and 27 extracting a string which is previously conditionally designated by RDB pseudo tables 13 and 26 on an output table 15 and an update table 28 and executing projection are provided. Data on the respective lines in table structure, which are held in the output table 15, are transmitted to an external part as EDI standard data format conversion, and data on respective lines in table structure, which are held in the update table 28, are processed as update data peculiar to the user after format conversion.