1. A method of detecting floating amount of original from an original table based on image information read by an image reading apparatus having the original table on which an original is placed, a line sensor, and a lens array arranged in a direction of arrangement of pixels of the line sensor, comprising: obtaining image information of the original as line image information by performing scanning in the direction of arrangement of pixels of the line sensor; extracting a distinguishing portion from the line image information; detecting a characteristic frequency by frequency analysis of image information of the distinguishing portion; and determining the floating amount of the original from the characteristic frequency.