Dot positions are measured by recording dots on a recording medium in a measurement line pattern including a plurality of lines of rows of the dots respectively corresponding to a plurality of recording elements. The measurement line pattern is read where a longitudinal direction of the plurality of lines is directed to a sub-scanning direction of an image reading apparatus. A plurality of averaging regions is allocated to different positions of line blocks, the lines arranged in a main scanning direction. The image signal is averaged in the allocated averaging regions, and average profile images are created for positions in terms of the main scanning direction. Positions of both edges of each line are determined according to the average profile images. Line positions in the averaging regions are determined according to the edge positions. Line positions in the line blocks are determined according to the line positions in the averaging regions.