1. An image processing apparatus for synthesizing an image from a plurality of images, comprising: a first synthesizing unit that determines one out of a plurality of images as a reference image, determines a first synthesis ratio based on correlation amounts between the reference image and the plurality of images excluding the reference image and synthesizes a first image from the plurality of images based on the first synthesis ratio; a noise reducing unit that reduces noise of at least the first image using a plurality of parameters and generates a plurality of noise-reduced images including at least a noise-reduced first image obtained as a result of reducing noise of the first image; and a second synthesizing unit that determines a second synthesis ratio according to the first synthesis ratio and synthesizes a second image from the plurality of noise-reduced images based on the second synthesis ratio.