1. An image processing method comprising: using an image processing apparatus to perform the following steps: a first step of detecting an iris region of an eye of a person from an original image; a second step of performing a predetermined image conversion to the iris region detected in the first step; and a third step of outputting the original image the iris region of which the predetermined image conversion has been performed to; wherein the predetermined image conversion is image conversion in which an image of the iris region is replaced with a predetermined iris pattern image which is an iris pattern image of a person different from the person or an artificial iris image artificially generated, the predetermined iris pattern image having been processed so that at least one of size, color and shape thereof is the same as the image of the iris region.