pattern will be described below with reference to fig3 fig3 illustrates a part of the adjustment chart shown in fig2 in the plural concentric circles pattern as a part of the adjustment chart the reference patterns and the adjustment patterns are alternately disposed adjacent to each other as shown in fig3 in this exemplary embodiment the number of the adjustment patterns plural concentric circles patterns is 5 the reference numerals 1 5 are respectively assigned for the respective adjustment patterns the reference patterns are printed by a few line screen so as to be resistant to the density variation in the image forming apparatus 1 the adjustment patterns are printed by an actual screen and the respective adjustment patterns are printed with different colors the original chart that is data for printing the adjustment chart will be described below with reference to fig4 fig4 illustrates image data of the original chart that is stored in the rom 4 as shown in fig4 four plural concentric circles patterns are disposed at upper left upper right lower left and lower right portions of the original chart each plural concentric circles pattern in the original chart will be described with reference to fig5 fig5 illustrates one plural concentric circles pattern in the image data of the original chart as shown in fig5 each plural concentric circles pattern includes doughnut shaped portions defined by contours of adjacent concentric circles in the plural concentric circles pattern in the plural concentric circles pattern as shown in fig4 and 5 doughnut shaped portions of the adjustment.