A process and apparatus for determining values of new pixels which are to be interpolated between original pixels of an image during magnification of the image. The interpolation is carried out in each unit area which is surrounded by original pixel centers. The process includes the following steps: a first step for detecting an angle of an original line image in a predetermined vicinity of the unit area by comparing values of a combination of original pixels in the predetermined vicinity of the unit area with each of a plurality of predetermined patterns of an arrangement of pixel values; a second step for selecting one of a plurality of predetermined modes of division of the unit area so that an angle of a dividing line which divides the unit area in the selected mode of division conforms to the angle detected in the first step; and a third step for determining a value of each of the new pixels by assigning a value which is determined based on the values of the original pixels in the vicinity to all of the new pixels the centers of which are in each section of the unit area which were generated by the mode of division selected in the second step.