Abstract: A self-diagnosing system for an encoder. The system includes an encoder circuit for outputting information detected by an encoder; a data preparing section provided in the encoder circuit and preparing data representing a state of an abnormal-state judgment factor in connection with at least one of the encoder and an electric motor combined with the encoder; a signal generating section provided in the encoder circuit and generating an abnormal-state signal when the data prepared in the data preparing section represents an abnormal state of the abnormal-state judgment factor; and a storage section provided in the encoder circuit and storing the data representing a state of the abnormal-state judgment factor when the signal generating section generates the abnormal-state signal. The storage section may store the data prepared in the data preparing section at a desired timing not later than the instant the signal generating section generates the abnormal-state signal.