From input document data there are detected the features of the document, such as the top, bottom, left and right margins, the character space, the line space, the word space, the column space, the size and coordinates of each character, and the like. Values by which the spaces thus detected, including the character space and the line space, are to be adjusted, and a ratio at which to reduce or enlarge the characters are calculated from the features of the document and the minimum margins, the minimum character size and the like preset based on the size of the medium used in an output device such as laser printer. The coordinates of each character on the document is changed in accordance with these values and the character-reducing or -enlarging ratio. The size of the input image is changed based on the original and changed coordinates of each character. The data representing the image thus changed is supplied to the output device, which display or prints a document image, with the spaces adjusted and the characters output in an optimum size.