1. An imaging device comprising: a single photographing optical system; an image sensor having a plurality of phase-different pixels for obtaining a plurality of viewpoint images by photo-electrically converting a luminous flux passing through different regions of the photographing optical system; a shading correction part for conducting a shading correction to the plurality of viewpoint images, and the shading correction part varies the amount of shading correction based on light-reduction property for one viewpoint image among the plurality of viewpoint images with respect to the amount of shading correction based on light-reduction property for another viewpoint image among the plurality of viewpoint images; and a stereoscopic image generating part for generating a stereoscopic image from the plurality of viewpoint images, wherein the image sensor includes a first pixel group a second pixel group corresponding to a first pupil splitting direction and a third pixel group and a fourth pixel group corresponding to a second pupil splitting direction perpendicular with respect to the first pupil splitting direction among each luminous flux passing through different regions of the photographing optical system, and the shading correction part varies the amount of shading correction based on light-reduction property for the viewpoint image corresponding to the first pixel group with respect to the amount of shading correction based on light-reduction property for the viewpoint image corresponding to the second pixel group, and varies the amount of shading correction based on light-reduction property for the viewpoint image corresponding to the third pixel group with respect to the amount of shading correction based on light-reduction property for the viewpoint image corresponding to the fourth pixel group.