PURPOSE: To easily ensure the compatibility of new and old user programs when extension data is newly added on a data return processing system transferring variable length data in a distributed processing system and the like.

CONSTITUTION: A request from an application processing part 11 is transmitted to a server processing part 22 through an interface processing part 15, and return data 23 of a processing result is returned from the server processing part 22. An application processing part 11A which requires extension data secures the return area of extension data and gives a data reception request. An application processing part 11B which does not require extension data gives the data reception request as in a conventional case. The interface processing part 15 executes an extension data return processing 19 or an extension data skipping processing 20 according to whether the return area where extension data is stored is secured or not.