A console acquires a displacement amount of an X-ray source from a displacement amount detector. In the case where an electronic cassette having an image detector is not appropriately positioned with respect to a body part to be imaged, a position of the X-ray source is changed in accordance with a position of the body part to be imaged. A dose measurement field position determining unit specifies a current relative position between the X-ray source and the electronic cassette having the image detector based on a displacement amount. The dose measurement field position determining unit determines a position of a dose measurement field based on the specified current relative position.
