1. An image forming apparatus including: an image forming unit adapted to form images of a plurality of colors to which a forgery discriminating pattern of a yellow color is added, the forgery discriminating pattern having a predetermined arrangement of dot toner images; a controller configured to control said image forming unit so as to form patch images, each of which is composed of a single color of the plurality of colors including yellow, magenta, cyan, and black, for density detection; a detector configured to detect a density of each of the patch images; and an adjusting unit for adjusting an image forming condition, wherein said controller controls said image forming unit so as to add the forgery discriminating pattern of the yellow color to all of the patch images, wherein said adjusting unit adjusts an image forming condition of the yellow color on the basis of a density of the patch image of the yellow color detected by said detector, and said adjusting unit adjusts an image forming condition of colors other than the yellow color on the basis of both the density of the patch image of the respective color other than the yellow color detected by said detector and a density of the forgery discriminating pattern of the yellow color, and wherein if (i) a first correspondence line indicates a relationship between gradation values and density values, obtained in a case where density control is executed based on the density of each of the patch images other than the yellow color detected by said detector without consideration of the density of the forgery discriminating pattern of the yellow color, in which said adjusting unit adjusts the image forming condition of colors other than the yellow color such that the densities are lighter than the ideal densities of the colors other than the yellow color, and (ii) a second correspondence line indicates a relationship between gradation values and density values, obtained in a case where density control is executed based on the density of each of the patch images other than the yellow color detected by said detector with consideration of the density of the forgery discriminating pattern of the yellow color, said adjusting unit adjusts the image forming condition of the colors other than the yellow color in a manner where a difference between the first correspondence line and the second correspondence line at a low-density portion is larger than the difference between the first correspondence line and the second correspondence line at a high-density portion.