An image signal processing apparatus for performing signal processing with respect to output from an image pickup device, which includes an image signal correcting unit for receiving an output signal corresponding to each of divided areas of the image pickup device to perform correcting processing. The image signal processing unit includes a calculating unit for calculating a total sum of pixel values of a string of pixels on a boundary between the divided areas, and further calculating a difference between the total sums of pixel values corresponding to the respective divided areas, and a correcting unit for making a comparison between the difference calculated by the calculating unit with a predetermined threshold, and then determining a correction mode of a divided area image based on the result of the comparison, followed by performing pixel value correcting processing with respect to the divided area image according to the determined correction mode.