1. An X-ray imaging apparatus characterized by comprising (A) an X-ray beam emitting device for emitting X-ray beams to an object under examination to be radiographed; (B) a two-dimensional X-ray detecting device for detecting transmitted X-ray images of the object under examination produced by emission of the X-ray beams to the object under examination, and outputting X-ray detection signals in real time; (C) a revolution device for revolving the X-ray beam emitting device and the two-dimensional X-ray detecting device about the object under examination as opposed to each other with the object under examination in between; (D) an X-ray energy switching control device for performing X-ray energy switching control to switch the X-ray beams emitted from the X-ray beam emitting device alternately to high energy X-ray beams of high energy and to low energy X-ray beams of low energy; (E) a high energy image acquiring device for acquiring high energy X-ray photo images based on X-ray detection signals outputted from the two-dimensional X-ray detecting device as a result of emission of the high energy X-ray beams; (F) a low energy image acquiring device for acquiring low energy X-ray photo images based on X-ray detection signals outputted from the two-dimensional X-ray detecting device as a result of emission of the low energy X-ray beams; (J) a 3D reconstruction device for carrying out a 3D reconstruction process based on the high energy X-ray photo images to acquire a 3D X-ray image of high energy X-ray radiography, and a 3D reconstruction process based on the low energy X-ray photo images to acquire a 3D X-ray image of low energy X-ray radiography; (K) a weight setting device for setting weights suitable for selecting a site of interest in the object under examination with respect to the 3D X-ray image of high energy X-ray radiography and 3D X-ray image of low energy X-ray radiography; and (L) an energy difference using subtraction device for acquiring a 3D subtraction image through an image subtraction process carried out, according to the weights set by the weight setting device, on the 3D X-ray image of high energy X-ray radiography and 3D X-ray image of low energy X-ray radiography.