1. A region-in-object measuring system comprising: a reference object having an object surface frame which is closely contacted with a surface of an object and a film frame which is closely contacted with an X-ray film, the two frames being separated from each other; and a computing device having: an input part which captures a two-dimensional X-ray image obtained by imaging a region-in-object by means of an X-ray imaging apparatus in a state in which the object surface frame of the reference object is closely contacted with the surface of the object; a first computing part which determines a projection scaling factor for an image of the object surface frame based on the size of a predetermined pattern on a plane of the object surface frame and the size of an image of a predetermined pattern in a two-dimensional X-ray image; a second computing part which determines a position vector of a specific point on the plane of the object surface frame and a position vector of an image of a specific point on the plane of the object surface frame in the two-dimensional X-ray image; a third computing part which determines a position vector of an X-ray source based on the projection scaling factor determined by the first computing part and the position vector of the specific point on the plane of the object surface frame and the position vector of the image of the specific point on the plane of the object surface frame determined by the second computing part; a fourth computing part which determines a position vector of an image of the region-in-object in the two-dimensional X-ray image; and a fifth computing part which determines a position vector of the region-in-object based on the position vector of the X-ray source determined by the third computing part and the position vector of the image of the region-in-object determined by the fourth computing part, and which decides an orientation of the region-in-object with respect to the location of the image region-in-object.