1. An image reading method of reading an image of an original by an image reading apparatus, comprising: a comparing step of comparing a value of a pixel of interest with that of a neighboring pixel of the pixel of interest, the pixel of interest being each pixel in image data corresponding to pixels arranged in a predetermined direction of the original; a first detecting step of detecting, as a first feature pixel, a pixel of interest having a value exhibiting a first feature compared to the value of a neighboring pixel, based on the comparison result in the comparing step; a second detecting step of detecting, as a second feature pixel, a pixel of interest having a value exhibiting a second feature compared to the value of a neighboring pixel, based on the comparison result in the comparing step; a first calculating step of calculating a first feature pixel to first feature pixel distance between neighboring first feature pixels detected in the first detecting step; a second calculating step of calculating a second feature pixel to second feature pixel distance between neighboring second feature pixels detected in the second detecting step; a third calculating step of calculating a feature pixel to feature pixel distance based on the first feature pixel to first feature pixel distance and the second feature pixel to second feature pixel distance; a histogram generating step of generating, from the feature pixel to feature pixel distances calculated from all pixels arranged in the predetermined direction, a histogram of a feature pixel to feature pixel distance having the same value in succession; a discriminating step of discriminating whether or not a feature pixel to feature pixel distance exceeding a predetermined frequency exists in the histogram; and a determining step of determining whether or not the original is a halftone original, based on the discrimination result in the discriminating step.