An FPD (26) detects an X-ray image of an object. The FPD (26) includes a plurality of pixels (40, 55) arranged in its image capturing field (41). Each pixel (40, 55) receives X-rays emitted from an X-ray source (16), and outputs a pixel value in accordance with an X-ray dose applied thereto. A pixel determiner (58) determines a minimum-value pixel out of the pixels (40, 55) based on the pixel values of the pixels (40, 55). The minimum-value pixel is a pixel whose pixel value is the lowest. The pixel determiner (58) sets the minimum-value pixel as an exposure control pixel. A comparator (59) compares a first integrated value, which is an integrated value of the pixel values of the minimum-value pixel, with a predetermined first threshold value. The comparator (59) performs X-ray emission control such that, when the first integrated value has reached the first threshold value, the X-ray source (16) stops emitting the X-rays.