1. An image processing apparatus comprising: an image sensor that is exposed to light, images, and outputs at least two images of a long accumulation image with a longer exposure time and a short accumulation image with a shorter exposure time in one field; a first long accumulation evaluation value computation unit that computes an exposure evaluation value from the long accumulation image and outputs the value as a first long accumulation evaluation value; a second long accumulation evaluation value computation unit that computes an exposure evaluation value from the long accumulation image according to a different method from that of the first long accumulation evaluation value computation unit, and outputs the value as a second long accumulation evaluation value; a short accumulation evaluation value computation unit that computes an exposure evaluation value from the short accumulation image and outputs the value as a short accumulation evaluation value; a short accumulation exposure control unit that acquires a short accumulation target exposure time and short accumulation exposure control information from the short accumulation evaluation value and a first target level; a long accumulation evaluation value synthesis unit that acquires a long accumulation evaluation value by synthesizing the first long accumulation evaluation value and the second long accumulation evaluation value in response to the short accumulation target exposure time using a predetermined method; a long accumulation exposure control unit that acquires long accumulation exposure control information from the long accumulation evaluation value and a second target level; and a synthesis unit that generates one image from the long accumulation image and the short accumulation image in response to the long accumulation exposure control information and the short accumulation exposure control information.