1. An image processing apparatus for synthesizing a long second exposure image derived by a long exposure time and a short second exposure image derived by a short exposure time, shorter than the long exposure time, for imaging of the same scene, the apparatus comprising: a long second middle luminance pixel selecting unit configured to select out a pixel having a pixel luminance value in a first middle luminance region from pixels included in the long second exposure image, based on comparing the pixel luminance value of the pixel in the long second exposure image with a first lower threshold value and with a first upper threshold value; a short second middle luminance pixel selecting unit configured to select out a pixel having a pixel luminance value in a second middle luminance region from pixels included in the short second exposure image, based on comparing the pixel luminance value of the pixel in the short second exposure image with a second lower threshold value and with a second upper threshold value; a correction gain factor calculating unit configured to designate, as a common pixel, the pixel selected by the long second middle luminance pixel selecting unit and the pixel selected by the short second middle luminance pixel selecting unit, each of the pixels being at a common position, and to calculate a correction gain factor based on the pixel luminance values of both of the pixels; and an image synthesizing unit configured to multiply a value of data representing the short second exposure image by the correction gain factor, and to synthesize the short second exposure image, the value of the data of which is subjected to the multiplication, and the long second exposure image.