1. A radiation imaging device, comprising: a radiation ray generation unit that generates radiation rays; an image detection unit that detects as an image radiation rays that are emitted by the radiation ray generation unit and pass through a subject; a subtraction image forming unit that forms a subtraction image by using detected images; and a control unit that, when the same subject is to be imaged consecutively plural times, changes an imaging time interval between imagings according to a set imaging region of the subject; wherein when the same subject is to be imaged consecutively plural times, the control unit controls the radiation ray generation unit so that a radiation ray tube voltage at each imaging increases monotonically, regardless of the set imaging region of the subject; and wherein the control unit determines which is output between an image obtained at a first imaging or an image obtained at a second imaging, according to the set imaging region of the subject.