1. A radiography apparatus comprising: a radiation source that outputs radiation to a subject; a detector that detects the radiation that has passed through the subject and produces an image of the subject; a body thickness information obtainment unit that obtains body thickness information about the subject from the image of the subject; a condition setting unit that sets, based on the body thickness information, a slice image obtainment condition representing a range in which a slice image is obtained in the subject; and a slice image obtainment unit that obtains, based on the slice image obtainment condition, the slice image, wherein the slice image obtainment unit obtains a plurality of radiographic images corresponding to a plurality of radiation source positions by moving the radiation source, relative to the detector, to the plurality of radiation source positions and by irradiating the subject with the radiation from the plurality of radiation source positions, and obtains the slice image of the subject by reconstructing the slice image from the plurality of radiographic images, and wherein the body thickness information obtainment unit obtains the body thickness information about the subject by analyzing the plurality of radiographic images.