1. An imaging device comprising: an imaging optical system including a focus lens; a lens movement mechanism that receives a focus operation to move the focus lens in an optical axis direction of the imaging optical system; an imaging element including an imaging surface provided with a matrix pixel array including a plurality of first pixel lines formed by a plurality of pixels including photoelectric conversion elements arranged in a first direction and a plurality of second pixel lines formed by the plurality of pixels arranged in a second direction perpendicular to the first direction, the plurality of pixels including first phase difference pixels and second phase difference pixels for selectively receiving object lights passed through different areas of the imaging optical system by pupil division and including normal pixels for receiving the object lights, the first direction parallel to a phase difference direction corresponding to a deviation direction of images of the object lights formed on the imaging surface by the imaging optical system, the second pixel lines provided with the plurality of first and second phase difference pixels; a first interpolation device obtaining outputs of first interpolation pixels positioned between the first phase difference pixels in the second pixel lines by an interpolation process based on outputs of the first phase difference pixels; a second interpolation device obtaining outputs of second interpolation pixels positioned between the second phase difference pixels in the second pixel lines by an interpolation process based on outputs of the second phase difference pixels; a photographic image generation device generating a photographic image from outputs of the normal pixels; a split image generation device using a first image formed by the outputs of the first phase difference pixels and the first interpolation pixels and a second image formed by the outputs of the second phase difference pixels and the second interpolation pixels to generate a split image; and a display device displaying the photographic image generated by the photographic image generation device, the display device displaying the split image generated by the split image generation device in a display area of the photographic image.