To prolong an exposure time in an image pickup apparatus that combines a plurality of images. A gain processing unit increases or decreases a plurality of image signals using gains different from each other. An analog to digital conversion unit generates a plurality of image data by performing analog to digital conversion on the increased or decreased plurality of image signals. A calculation unit adds data obtained by multiplying short-time exposure data being any of the plurality of image data, by a ratio between the gains, and data among the plurality of image data that does not correspond to the short-time exposure data, and outputs the added data as long-time exposure data. A combining unit combines the short-time exposure data and the long-time exposure data at a predetermined combining ratio.