1. An image processing apparatus which generates, from a first image obtained by shooting and a second image obtained by shooting with an exposure time longer than an exposure time of the first image, a third image in which noise in the first image has been reduced and a fourth image which is a synthesized image of the second and third images, the image processing apparatus generating an output image by synthesizing the first and fourth images, the image processing apparatus comprising: a synthesis ratio adjustment portion which adjusts, according to a noise level in the first image, at least one of a first synthesis ratio at which the second and third images are synthesized and a second synthesis ratio at which the first and fourth images are synthesized.