An image forming apparatus that includes: an image reading portion capable of reading image data of the document sheet; document sheet detection sensors; an ADF including a document holding surface on which first and second areas having different reflection characteristics and third and fourth areas having different reflection characteristics are formed; a first reading control portion that causes the image reading portion to read image data of both the first and second areas when the ADF is closed; a first document sheet width detection portion that detects a width of the document sheet based on the image data read from the first area or the image data read from the second area; and a first size detection portion capable of detecting the size of the document sheet based on detection results from the first document sheet width detection portion and the document sheet detection sensors.