1. An imaging device comprising: a pixel region comprising a plurality of pixel elements, for imaging as an image based on an incident light; and a reading unit for reading out a thinned out image in low resolution by thinning out a pixel element from the pixel region, and for reading out a partial image in higher resolution than the resolution of the thinned out image from a partial region of the pixel region, wherein the reading unit reads out the thinned out image and the partial image from respectively different pixel elements, and from respectively different imaging frame.