1. An image capturing apparatus comprising: an image sensor including a plurality of pixels arranged two-dimensionally; a light-shielding unit configured to shield said image sensor from light; a composition unit configured to perform continuous shooting of a plurality of exposure images by said image sensor and compose the plurality of exposure images obtained by the continuous shooting; a control unit configured to perform control to acquire a first black image before the continuous shooting by causing said image sensor to accumulate electric charges while said image sensor is shielded from light by said light-shielding unit and to acquire a second black image after the continuous shooting by causing said image sensor to accumulate electric charges while said image sensor is shielded from light by said light-shielding unit; and an image processing unit configured to perform first noise reduction processing using the first black image and second noise reduction processing using the second black image for each of a plurality of exposure images obtained by the continuous shooting or an exposure image obtained by composing the plurality of exposure images by said composition unit.