1. An image processing apparatus comprising: a first holding unit configured to hold acquired images that are sequentially acquired; a first synthesis unit configured to synthesize a first set number of acquired images among the acquired images held in said first holding unit to generate first composite images; a second holding unit configured to 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 an deleting unit configured to sequentially delete the acquired images used to generate the first composite images.