A region-in object measuring system comprises a reference object (1) having two frames, i.e., an object surface frame (2) brought into contact with the surface of an object and a film frame (3) with which an X-ray film (7) is brought into contact, spaced from each other and a computing device. The computing device sets a three-dimensional X-ray coordinate system using the reference object (1) according to a two-dimensional X-ray image acquired by imaging a region in an object in a state that the object surface frame (2) of the reference object (1) is in close contact with the surface of the object by means of an X-ray imaging apparatus, determining the position of an X-ray source from the projection magnification and size of the image of the object surface frame (2), determining the position vector of a specific region in a body according to the position of the X-ray source, and accurately determines the direction and three-dimensional position of the region in the object with respect to the position of the image of the region in the object shown on the X-ray image.