1. An apparatus for forming a parenthesized image in which a parenthesized image constituting one of a front and rear pair of images of an arbitrary height and arbitrary line width is formed based on outline font, said apparatus comprising: means for storing outline font data as original outside shape data, said original outside shape having a single closed curve made up of a parenthesized outside shape portion of the parenthesized image and a connecting line portion to connect both open ends thereof; means for determining the height and line width of the parenthesized image; means for forming, based on the outside shape data, a first-size solidly shaded image which has the determined height and is entirely filled inside the original outside shape thereof with effective pixels of logic “1”; means for forming, based on the outside shape data, a second-size solidly shaded image of a second size which is smaller by the line width than the first-size solidly shaded image, along the parenthesized outside shape portion; and means for performing non-equivalence operation with corresponding pixels by displacing the first-size solidly shaded image and the second-size solidly shaded image by the line width along the parenthesized outside shape portion.