1. An imaging device that produces image data showing an image with a target resolution lower than an original resolution of the imaging device, comprising: a plurality of photoelectric conversion elements which are arranged in a two-dimensional matrix, each have a color filter of a predetermined color selected from a plurality of colors on a light-receiving surface thereof, and each store an electric charge by photoelectric conversion; a charge adding unit operable to, for each portion of L rows and C columns in the matrix, where 6≦L, 6≦C and L and Care even natural numbers, add together electric charges stored in photoelectric conversion elements that have color filters of a same color in the portion, to obtain a resulting electric charge; a read unit operable to read resulting electric charges for all portions of L rows and C columns in the matrix; an image data generating unit operable to generate image data showing an image based on the read resulting electric charges, the image having a tentative resolution which is lower than the original resolution; and a zoom unit operable to zoom the image with the tentative resolution so as to reduce the tentative resolution to the target resolution and produce the image data showing the image with the target resolution, wherein a number of the plurality of photoelectric conversion elements is greater than a number of photoelectric conversion elements required for the tentative resolution to be equal to the target resolution.