Abstract: According to an embodiment, a radiation therapy device includes an acquirer, a projection position calculator, an element projection image generator, and an element projection image synthesizer. The acquirer acquires a condition of X-ray imaging in a treatment stage and a three-dimensional image of a patient imaged before the treatment stage. The projection position calculator calculates a projection position when each of pixels included in the three-dimensional image is projected onto a two-dimensional X-ray fluoroscopic image generated in the X-ray imaging on the basis of the condition of the X-ray imaging. The element projection image generator generates an element projection image for each pixel when each of the pixels included in the three-dimensional image is projected onto the X-ray fluoroscopic image. The element projection image synthesizer performs a synthesis process for the element projection image for each pixel on the basis of the projection position to generate a reconstructed image.