An image obtaining unit obtains a plurality of projection images by imaging a subject with different radiation source positions. A pixel value projecting unit projects pixel values of the projection images on coordinate positions on a desired slice plane of the subject based on the positional relationship between the radiation source position with which each projection image is taken and the radiation detector, while preserving pixel values of the projection images, to obtain a plurality of slice plane projection images. A positional misalignment correcting unit corrects positional misalignment between the slice plane projection images. A pixel value calculating unit generates a tomographic image from the slice plane projection images having been subjected to the correction of the positional misalignment.