A method of automatic data input/output to/from another document format or data file with a high visibility and an easy document format definition operation, even in a document format having a derivative area as a second data description area in a single cell. The method includes performing an item definition for correlating data or a data field with an item name in a document format, and specifying a difference condition as a description condition unique to the derivative area against a main area as a first description area in the same cell, for the data field having the derivative area as a second data area in a single cell. According to the specified item definition and the difference condition, data or a record as a set of data description areas to be simultaneously referenced at the document data input/output is identified for each of the derivative areas.