An image forming apparatus determining a paper feed source having a possibility of feeding paper sheets in executing a printing job in accordance with contents of the printing job and detecting characteristics of a paper sheet fed from the paper feed source for executes printing processing of a printing job, wherein the printing of the printing job is performed by feeding at least one paper sheet from each of all the paper feed sources having a possibility of paper feed and, if the detected characteristics of the paper sheet do not meet the characteristics of a paper sheet stored in a memory, printing processing is temporarily or completely stopped.