The invention can prevent problems due to the feeding of an OHT sheet for ink jet, such as damaging a fixing device or deterioration of the image, as well as damaging of the fixing device due to the feeding of a sheet with a thickness exceeding a regular thickness, and can transmit related information to the user, so as to enhance reliability and usability of the image forming apparatus. The image forming apparatus includes a feeding unit for feeding a recording material, an image forming unit for forming an image on the recording material fed by the feeding unit, a reading unit capable of reading an image of an area including a portion of a surface of the recording material fed by the feeding unit, a determining unit for determining whether or not the recording material is an irregular recording material, based on the image read by the reading unit, and a controlling unit for stopping or suppressing a specific operation of the apparatus, when it is determined by the determining unit that the recording material is an irregular recording material.