1. An imaging device comprising: a pixel region that includes a plurality of pixel elements for imaging an object based on an incident light; and a reading unit for reading out a thinned out image at a low resolution by thinning out a pixel element from the pixel region, and for reading out a partial image at a higher resolution than the low resolution of the thinned out image, the reading unit reading out the partial image from a partial region of the pixel region, wherein the reading unit reads out the thinned out image in a first frame from a first set of the plurality of pixel elements and reads out the partial image in a second frame from a second set of the plurality of pixel elements excluding the first set of the plurality of pixel elements, and wherein the first and second frames are consecutive frames.