Reference image signals representing at least two reference images of different phases of an object, whose phase varies within a predetermined variation range, are stored. Successive image recording operations are performed on the object, whose phase is varying within the predetermined variation range, with a low radiation dose and at predetermined time intervals, and successive recorded images are acquired. The phase of the object is detected for each of the successive recorded images and in accordance with the reference images of the different phases of the object. When the detected phase of the object coincides with a desired phase, an ordinary image recording operation with an ordinary radiation dose is performed.