An image processing apparatus obtains information of an effective pixel of an image pickup element and information of a pixel on the periphery thereof, sets a position where image data of the peripheral pixel is multiplexed in accordance with a transmission method, generates a data stream by multiplexing image data of the effective pixel and the image data of the peripheral pixel in accordance with the set multiplexing position and the transmission method, and multiplexes information on the set multiplexing position and information on the peripheral pixel area to the data stream.