An optical printer which exposes photosensitive paper (500) at a preset timing using light from a light source (110) moving relatively to the photosensitive paper (500) and based on image data comprising pixels having a plurality of gradations which linearly vary from a low-color-density minimum gradation to a high-color-density maximum gradation, wherein an area gradation is effected by adjusting an exposure distance to form a gradated image on the photosensitive paper (500). A gradation does not change in proportion to an exposure distance over a whole range but changes non-linearly.
