1. A method of determining whether or not to perform a decoloring process on a sheet, the method comprising the steps of: detecting a front position of a sheet fed in a sheet transport direction; detecting a presence or absence of an identification mark on a front portion of the sheet outside of an image forming area of the sheet, after detecting the front position of the sheet; determining whether an image formed with decolorable color material is printed on the sheet based on whether an identification mark is detected on the front portion of the sheet; determining whether to perform decoloring of the image based on the determination of whether an image formed with decolorable color material is printed on the sheet; and when the image formed with decolorable color material is determined to be printed on the sheet, decoloring the image.