An image processor which is capable of performing correction of defective pixels without degrading image quality when synthesizing a plurality of still images. When a plurality of image data items are synthesized, a first reference value which is smaller than a second reference value for use in determining whether or not to correct pixel data forming the image data is compared with a pixel value indicated by each of synthesized pixel data items forming the synthesized image data, and first correction processing is performed in which the synthesized pixel data of the synthesized image data is corrected according to a result of comparison.