PURPOSE: To reduce other input field by enlarging the input field concerned, in a state that the whole input screen, that is, size of one page remains held constant, in the case data inputted to a certain input field is not held in its field, in the input screen constituted of plural input fields.

CONSTITUTION: An input field whose enlargement is desired, is designated from an input device 12. A screen reconstituting part 13 reads out the contents of an input field to be enlarged from a screen defining information storage part 15 and a present screen state storage part 16, and also, detects an input field to be reduced. The contents of the input field to be enlarged and reduced of the present screen state storage part 16 are updated, and the updated contents are converted to graphic data by a data converting part 14 and displayed on a display device 11. In such a way, while holding size of the hole screen constant, size of the input field can be changed freely.