An image forming apparatus includes an image reading device, a printing device, a specific character string detector, and a controller. The specific character string detector extracts a specific area image, enlarges the specific area image, executes character recognition processing on the enlarged specific area image to detect a character string, and determines whether or not the detected character string includes a predetermined specific character string. The controller prohibits copying of the document image when the predetermined specific character string is included in the detected character string. The size of the specific character string before enlargement is smaller than a lower limit character string size of character recognition in the character recognition processing, and the size of the specific character string after enlargement is equal to or larger than the lower limit character string size of character recognition in the character recognition processing.