An X-ray imaging system includes a plurality of electronic cassettes having FPDs. One of the plural electronic cassettes for use in imaging is selected by input operation. Control is performed in a first mode operation being normal for the electronic cassette selected in the selecting step. Control is performed in a second mode operation being auxiliary of which driving power is lower than in the first mode operation for one of the electronic cassettes in an unselected state in the selecting step. Assuming that irradiation of the radiation is detected in the second mode operation, error information is displayed in relation to selecting the electronic cassette in the selecting step. For example, the first and second mode operations are pixel reset in which charge stored in pixels are swept in an FPD.