PURPOSE: To display reading and a KANJI(Chinese character) mixed text performing KANA(Japanese syllabary)/KANJI conversion processing at the input position of a document by providing an in-line display control means which inserts a null area to a character string on display temporarily and displays the reading and the KANJI mixed text on the null area.

CONSTITUTION: A KANA/KANJI conversion means 501 decides whether or not delivered data shows a key to instruct the start of KANA/KANJI conversion, and stores an input start position when it is the key to instruct the start of the KANA/KANJI conversion, and awaits the next key input. The means inserts the null area to a stored input start position temporarily when it is not a definition key. In such a case, null data is not inserted to an actual document, and document data considered as that the null data is inserted to a virtual document of the same image as that of the actual document is displayed on a display device 106. Thereby, the movement of a line of sight between the document and a conversion display area can be eliminated, and the fatigue of the eyes can be reduced, and also, the preparing efficiency of the document can be improved.