There is disclosed a mask pattern data forming method comprising arranging patterns with a minimum pitch in parallel or vertically in an X or Y direction, where directions diagonally connected with respect to respective centers of gravity of the double-pole or the quadrupole illumination are defined as the X and Y direction, classifying patterns or pattern groups with a pitch larger than the patterns with the minimum pitch into a pattern type with a pitch whose exposure margin is larger than that of the patterns with the minimum pitch and a pattern type with a pitch whose exposure margin is smaller than that of the patterns with the minimum pitch; and arranging patterns or pattern groups classified into the pattern type whose exposure margin is smaller than that of the patterns with the minimum pitch in a direction deflected by 45° or 135° from the patterns with the minimum pitch.