PURPOSE: To reduce the registering capacity of a zip code conversion file in the case of converting an address data into a zip code by using the zip code conversion file.

CONSTITUTION: The address data of a part in which the zip code 1-2 can be judged at least is recorded in the conversion index of a new zip code conversion file 1-4, and the blank part of the address data 1-1 of the conversion index sets the largest character code in a computer system, and further, the conversion index are arranged in order from the smaller characters of the computer system to be used to the larger one, and data to coincide with the address data up to the blank part of the conversion index registered in the new zip code conversion file is retrieved by a zip code conversion control part 1-3, thereby the address data is converted into the zip code based on the zip code of a record which could be retrieved.