An X-ray image forming device comprising: an X-ray imaging unit including a rotating member rotatable about a rotating shaft and linearly movable, and an X-ray source and an X-ray sensor disposed at opposite ends of the rotating member to face each other with a region of interest therebetween; a penetration data acquisition unit configured to acquire X-ray penetration data from multiple directions crossing through locations corresponding to an image layer in the region of interest by controlling the X-ray imaging unit, wherein an imaging trajectory set throughout the region of interest is divided into multiple sections, wherein the X-ray source is adapted to emit an X-ray beam at a number of irradiation times, while the rotating member rotates along the multiple sections of the image trajectory; and wherein the device is adapted such that the imaging density may have a different value from section to section, the imaging density being a ratio of irradiation times of X-ray beam to a length of the imaging trajectory; and an image reconstructor configured to generate projection data by location within a predetermined angle range at each section of the image layer from the X-ray penetration data acquired by crossing through locations corresponding to the image layer in the region of interest, and reconstruct a two-dimensional X-ray panoramic image of the image layer based on the projection data by location, wherein the image reconstructor reconstructs the two-dimensional X-ray panoramic image by backprojecting the projection data by location for each of locations corresponding to the image layer at each section of the image layer, wherein the region of interest includes a dental arch, the image layer is within the dental arch, and the predetermined angle range gradually increases from a molar tooth to an anterior tooth. an X-ray imaging unit including a rotating member rotatable about a rotating shaft and linearly movable, and an X-ray source and an X-ray sensor disposed at opposite ends of the rotating member to face each other with a region of interest therebetween; a penetration data acquisition unit configured to acquire X-ray penetration data from multiple directions crossing through locations corresponding to an image layer in the region of interest by controlling the X-ray imaging unit, wherein an imaging trajectory set throughout the region of interest is divided into multiple sections, wherein the X-ray source is adapted to emit an X-ray beam at a number of irradiation times, while the rotating member rotates along the multiple sections of the image trajectory; and wherein the device is adapted such that the imaging density may have a different value from section to section, the imaging density being a ratio of irradiation times of X-ray beam to a length of the imaging trajectory; and an image reconstructor configured to generate projection data by location within a predetermined angle range at each section of the image layer from the X-ray penetration data acquired by crossing through locations corresponding to the image layer in the region of interest, and reconstruct a two-dimensional X-ray panoramic image of the image layer based on the projection data by location, wherein the image reconstructor reconstructs the two-dimensional X-ray panoramic image by backprojecting the projection data by location for each of locations corresponding to the image layer at each section of the image layer, wherein the region of interest includes a dental arch, the image layer is within the dental arch, and the predetermined angle range gradually increases from a molar tooth to an anterior tooth.