An image forming unit forms an image on a sheet. A first generation circuit generates, regarding a member involved in image formation in the image forming unit, a first signal relating to a first abnormality. A second generation circuit generates, regarding the member, a second signal that relates to a second abnormality different from the first abnormality, and whose feature in a time axis direction or a frequency axis direction is different from that of the first signal. A control circuit has a single port to which the first signal and the second signal are to be input. The control circuit identifies between the first signal and the second signal based on a feature of a signal input to the port.