1. A radiation imaging apparatus comprising: a table formed with an opening for allowing a breast of an examinee to pass through when the examinee lies down on the table; a radiation generating unit for applying a radiation beam to the breast passing through the opening of said table; a radiation detecting unit for detecting the radiation beam applied by said radiation generating unit and transmitted through the breast to output detection signals; a supporting plate for supporting said radiation generating unit and said radiation detecting unit such that said radiation generating unit and said radiation detecting unit are positioned to face each other with a rotational axis in between, said rotational axis being substantially orthogonal to said table and passing through said opening; a rotational driving device for rotating said supporting plate around the rotational axis; image processing means for generating image signals representing a predetermined number of radiation images based on the detection signals outputted from said radiation detecting unit and generating a tomographic image signal representing a radiation tomographic image by reconstructing the predetermined number of radiation images in a first imaging mode for obtaining the radiation tomographic image, and for generating a planar image signal representing a radiation planar image based on the detection signals outputted from said radiation detecting unit in a second imaging mode for obtaining the radiation planar image; computing means for computing an imaging direction for obtaining the radiation planar image in the second imaging mode based on a location designated in the radiation tomographic image obtained in the first imaging mode; control means for controlling said rotational driving device and said radiation generating unit to perform imaging for obtaining the radiation tomographic image in the first imaging mode, and controlling said rotational driving device according to the imaging direction computed by said computing means and controlling said radiation generating unit to perform imaging for obtaining the radiation planar image in the second imaging mode; and an image display unit for selectively displaying at least one of the radiation tomographic image obtained in the first imaging mode and the radiation planar image obtained in the second imaging mode.