A diagnostic method and apparatus for self-diagnosis a load side of an electric device to detect the occurrence of the abnormal electricity usage at the load using a simple apparatus based on a current sensor includes checking rated power consumption of a load, measuring power consumption of the load in operation, calculating a change in the power consumption by comparing the rated power consumption of the load with the power consumption of the load in operation, and determining that an abnormal power consumption occurs at the load when the change in the power consumption is outside a predetermined range.