Proper correction data is set by scanning a test chart even if the test chart has pixel lines formed thereon as inclined with respect to a secondary scanning direction. A virtual line CL is determined in a middle position in a primary scanning direction on the test chart, which extends between center indices 43C and 44C at opposite ends in the secondary scanning direction of the test chart. A scanning lines SL extending parallel to the virtual line CL is set to each pixel line Q. Sampling areas SQ are set on the scanning line SL. Density data is acquired from a plurality of sampling points formed in each sampling area SQ.