1. An image processing method for identifying, from a binary image, a plurality of pixel regions where pixels determined to be having the same pixel values are adjacent to each other, extracting, based on a pixel coordinate value of the identified pixel region, a feature point of the pixel region, calculating features of an image based on the extracted feature point, and determining similarity among images based on the calculated features, the method comprising steps of: deciding a neighboring feature point located within a surrounding region of the extracted feature point; counting the number of the decided neighboring feature points; determining whether the counted number of the feature points is equal to or lower than a second threshold value; calculating the features based on the neighboring feature point when the counted number of the feature points has been determined to be above the second threshold value; and changing a range of the surrounding region when the counted number of the feature points has been determined to be equal to or lower than the second threshold value.