An abnormality detecting method and an image processing method for an image processor is provided. The image processor can easily detect an abnormal rotation of a photosensitive drum without having a special detecting means but using an existing sensor, and keeps image data until a normal toner image is transferred and fine printing is carried out. The image processor forms a toner image corresponding to image data on a plurality of photosensitive drums, and detects the position of the toner image at a given part on a transfer object, to which given part the toner image formed on the photosensitive drum is transferred, to detect a rotation failure of the photosensitive drum. When a rotation failure is detected, the image processor keeps input image data without deleting even after the end of image formation.