1. A projection system constructed such that respective images corresponding to a first image light and a second image light can be stacked on a projection surface for display, comprising: a first image forming unit which modulates light emitted from a light source based on image data and releases the modulated image light as the first image light; a second image forming unit which modulates light emitted from the light source based on image data and releases the modulated image light as the second image light; a polarization combining system which combines the first image light released from the first image forming unit and the second image light released from the second image forming unit; a projection unit which projects the first image light and second image light combined by the polarization combining system on the projection surface; a pixel shift control unit which controls a pixel shift unit configured to perform pixel shifting such that the respective images corresponding to the first image light and the second image light can be relatively shifted on the projection surface by a predetermined amount with respect to each other; a display timing control unit which controls display timing such that display timing of the respective images corresponding to the first image light and the second image light can be shifted by a predetermined period; and an image display control unit configured to judge whether the respective images corresponding to the first image light and the second image light are still images or dynamic images and also configured to control, based at least partly on the judgment of the image display control unit, the pixel shift control unit and the display timing control unit, wherein the image display control unit is configured to perform only pixel shifting control for the pixel shift control unit, only display timing control for the display timing control unit, and both the pixel shifting control for the pixel shift control unit and the display timing control for the display timing control unit, and wherein the image display control unit is configured to perform the pixel shifting control for the pixel shift control unit when the respective images corresponding to the first image light and the second image light are still images, and is configured to perform at least display timing control for the display timing control unit when the respective images are dynamic images.