An image processing apparatus comprises a boundary line position storage section storing position information on boundary lines for dividing an image according to an image data unit into regions; a position information changing section to change the position information on boundary lines; and a dividing section to divide the image data unit into partial image data units corresponding to the respective regions. When a boundary line stored in the boundary line position storage section is across a principal part of the image, the position information changing section changes the position of the boundary line so as not to be across the principal part. When the regions divided by the boundary lines stored in the boundary line position storage section, includes a region that exceeds the maximum print size, the position information changing section changes the positions of boundary lines so that any region does not exceed the maximum print size.