An image forming apparatus includes a plurality of types of parts, and forms an image on a sheet using the plurality of parts. The apparatus includes the following. An image former forms the image on the sheet. An image reader reads image data of the sheet on which the image is formed. A hardware processor detects an image abnormality from the read image data, specifies a target part for diagnosis from the plurality of parts based on the detected image abnormality, calculates a deterioration degree of the specified target part, and diagnoses whether the target part is a cause for the detected image abnormality based on the calculated deterioration degree.