1. An image pickup apparatus comprising: an image pickup device in which a plurality of unit pixels are arranged in a two-dimensional matrix form, each of the plurality of unit pixels including: a plurality of first sensitivity pixels, which are arranged at a periphery of the unit pixel, each first sensitivity pixel configured to have a first sensitivity based on a first light receiving area; and at least one second sensitivity pixel configured to have a second sensitivity based on a second light receiving area smaller than a total area of the first light receiving areas of the plurality of first sensitivity pixels included in the unit pixel, wherein the first light receiving areas of each of the first sensitivity pixels of each unit pixel differ in size and form from the second light receiving area of the second sensitivity pixel of the respective unit pixel; an adding circuit configured to add pixel signals that are outputs of the plurality of first sensitivity pixels such that the centroid position of the total area of the first light receiving areas of the plurality of first sensitivity pixels included in the unit pixel approximately coincides with a centroid position of the second light receiving area of the at least one second sensitivity pixel; and a correction circuit configured to correct an added first sensitivity pixel signal having undergone addition by the adding circuit and a second sensitivity pixel signal of the at least one second sensitivity pixel based on preset common shading correction data.