An image forming apparatus sets, on an image forming unit, one of a first transfer condition that does not cause an image to be formed on a sheet and a second transfer condition that causes an image to be formed on the sheet. The apparatus generates a first reading result by reading the sheet that has passed through the image forming unit set in the first transfer condition, and generates a second reading result by reading the sheet that has passed through the image forming unit set in the second transfer condition. The apparatus detects toner fogging based on the first reading result and a reading result out of the second reading result, the reading result concerning a non-image region in which no image is formed on a surface of the sheet.