1. An imaging device comprising: an image acquisition unit that acquires an original image in which pixels with different exposure periods coexist; a corrected-image generating unit that generates corrected images individually for a plurality of pixel groups formed by classifying the pixels of the original image according to the exposure periods, the corrected images being generated by interpolating pixel values and executing exposure correction; and an HDR combining unit that combines the corrected images respectively generated for the plurality of pixel groups by the corrected-image generating unit, wherein, in a case where a difference between the pixel value of a pixel in the corrected image belonging to one of the pixel groups, in which the pixel value is acquired by exposure, and the pixel value of a pixel in the corrected image belonging to another one of the pixel groups, in which the latter pixel value is generated by interpolation at the same position as the former pixel, is greater than or equal to a predetermined threshold, the HDR combining unit increases a combining ratio by which the pixel in the corrected image belonging to the one of the pixel groups is combined.