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; determining whether the sheet is skewed based on the detection of the front position of the sheet; 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 that an image formed with decolorable color material is printed on at least one side the sheet if the presence of the identification mark is detected on the front portion of the sheet; determining that an image formed with decolorable color material is not printed on at least one side of the sheet if the absence of the identification mark is detected on the front portion of the sheet; and if the sheet is determined to not be skewed and if the image formed with decolorable color material is determined to be printed on the at least one side of the sheet, decoloring the image.