1. A computer tomography apparatus comprising: an x-ray source configured to emit x-rays while rotating around an object to be examined; an x-ray detector having x-ray detection elements arranged in a plurality of rows along a slice-thickness direction, each row having a number of x-ray detection elements configured to detect x-rays passed through the object and produce projection data of the object; a data acquisition device configured to acquire the projection data; a first reconstruction device configured to reconstruct an image from the acquired projection data assuming the x-rays intersect perpendicularly with an axis of rotation of the x-ray source so as to produce a fan-beam reconstructed image; a second reconstruction device configured to reconstruct a image from the acquired projection data using x-ray cone angle information so as to produce a cone-beam reconstructed image, a setting device configured to select a predetermined number of slices to be analyzed; and a reconstructing device selector configured to select, in correspondence to the number of slices selected, one of the first reconstruction device and the second reconstruction device.