The continuous image capturing of a subject is performed with small doses of radiation. A plurality of auxiliary images obtained by the continuous image capturing is stored. On the basis of the stored auxiliary images, the periodicity of motion of the subject is detected. A pseudo image is generated from the auxiliary images exhibiting the detected periodicity. The generated pseudo image is analyzed. On the basis of the analysis result, an image capturing parameter used for the main image capturing of a still image of the subject is calculated. Using the calculated image capturing parameter, the main image capturing of the still image of the subject is performed.