1. An image processing apparatus comprising: a pattern producing unit producing, based on a provided first pattern image, a second pattern image having substantially the same shade level as said first pattern image; and a latent image producing unit producing an image containing a latent image by selectively combining said first pattern image and produced said second pattern image according to values of respective pixels of a mask image provided for producing said latent image, wherein said first pattern image is formed of a plurality of information patterns bearing different items of assigned information, the image processing apparatus further comprising: an image expanding unit effecting expansion processing on provided said first pattern image, and providing said first pattern image subjected to the expansion processing to said pattern producing unit, and wherein said pattern producing unit converts a basic pattern image to said second pattern image by replacing a value of each of pixels of said basic pattern image prepared in advance with a value of a logical product between the value of said pixel and a value of the corresponding pixel of said first pattern image, wherein both of said first and second pattern image comprise the background of said image containing a latent image.