1. An image processing apparatus comprising: a first holding unit configured to hold a first set number of acquired images that are sequentially acquired; a first synthesis unit configured to sequentially synthesize the first set number of newly held acquired images to generate first composite images; a second holding unit configured to sequentially hold the first composite images; a second synthesis unit configured to synthesize images from an image generated last to an image corresponding to a second set number among the first composite images held in said second holding unit to generate a second composite image; and a generating unit configured to generate moving image data using the second composite images based on a moving image format, wherein said second holding unit is capable of further holding a generating condition for use in generating the first composite image, and wherein when said second holding unit holds the generation condition, said second synthesis unit generates the second composite image based on a synthesis condition determined with consideration given to the generating condition, the generating condition comprising a plurality of ISO sensitivities.