Image forming apparatus that detects occurrence of image defect

An image forming apparatus, comprising:

• an image forming unit configured to form an image on a sheet with use of toner based on original image data transmitted from an external device;
• a selection unit configured to determine a non-image region of the sheet to which the toner does not attach in a formation of the image by the image forming unit based on the original image data, and select, from the non-image region, a determination region depending on an image defect to be detected;
• a reading unit configured to read a surface of the sheet;
• and
• a control unit configured to obtain read image data by causing the reading unit to read the determination region after the image forming unit has formed the image, and perform detection processing for detecting whether the image defect has occurred in the formation of the image on the sheet based on the read image data.