1. An image processing apparatus for synthesizing a long second exposure image derived by larger exposure quantity and a short second exposure image derived by smaller exposure quantity, for imaging of the same scene, comprising: a long second middle luminance image detecting unit for detecting a pixel position of a pixel having a pixel value in first middle luminance pixel value region from the long second exposure image; a short second middle luminance image detecting unit for detecting a pixel position of a pixel having a pixel value in second middle luminance pixel value region from the short second exposure image; a correction gain factor calculating unit for designating, as a common pixel, a pixel having a common pixel position such that the pixel position detected by the long second middle luminance image detecting unit is the same as the pixel position detected by the short second middle luminance image detecting unit, and for calculating a correction gain factor based on the pixel value of the long second exposure image and the pixel value of the short second exposure image at a pixel position of the common pixel position; and an image synthesizing unit for synthesizing for multiplying the short second exposure image by the correction gain factor, and for synthesizing the short second exposure image subjected to the multiplication and the long second exposure image.