1. An image processing apparatus for reconstructing a tomographic image of a subject based on a plurality of projection images obtained by detecting, by a detection unit, radiation emitted from a plurality of different positions, comprising: a CPU; and at least one memory, the CPU and the at least one memory cooperating to provide: a reconstruction unit configured to reconstruct a first tomographic image from the plurality of projection images; an extraction unit configured to extract a fixed pattern occurring in the first tomographic image due to the detection unit; an update unit configured to form a second tomographic image by updating the first tomographic image using a value concerning intensity of the fixed pattern; and an output unit configured to output, as the tomographic image of the subject, the second tomographic image after update, wherein the update unit updates the first tomographic image based on differences between the projection image of the first tomographic image before update and a projection image of the second tomographic image after update.