A detector for detecting an abnormality in the developer passage includes a sensing unit for detecting a rotational speed of the developer pump, a rotational speed storage for storing rotational speed reference of the developer pump, and a controller for controlling the driving of the developer pump and detecting the abnormality in the developer passage on the basis of a comparison between a rotational speed calculation of the developer pump according to a detection signal from the sensing unit and a rotational speed reference of the developer pump stored in the rotational speed storage. The detector can be advantageously used for detecting a leak and/or a clogging in the developer passage caused by a manufacture/assembly defect, or sediment of the developer sludge, and can inform the user about the abnormal state of the developer passage.