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 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 as a regularization term; and an output unit configured to output, as the tomographic image of the subject, the second tomographic image obtained by the update.