1. A scan planning method for obtaining a tomographic image in a computed tomographic system, comprising: a first process for acquiring, along different directions, projection data of a plurality of parallel slices of an object to be examined and for reconstructing a tomographic image of each of said plurality of parallel slices; a second process including interpolating the plurality of tomographic images obtained by the first process and for obtaining three-dimensional image data of a region defined between two outermost slices of said plurality of parallel slices; a third process for forming and displaying a three-dimensional object image in accordance with said three-dimensional image data obtained by the second process and for changing the viewpoint of said three-dimensional object image in accordance with externally entered first coordinate information; a fourth process for superposing a slice position image on said three-dimensional object image displayed in the third process, said slice position image representing a slice position, and for changing the position and orientation of said slice on a screen in accordance with externally entered second coordinate information; and a fifth process for acquiring, along different directions, projection data of said slices corresponding to relative position information between said three-dimensional object image and said slice position image which are obtained in the third and fourth processes, and for reconstructing tomographic images of said respective slices.