1. A projection system constructed such that 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 for performing 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; 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 having a function which controls the pixel shift control unit and a function which controls the display timing control unit, wherein the image display control unit performs the pixel shifting control for the pixel shift control unit when the image is a still image, and performs at least display timing control for the display timing control unit out of the pixel shifting control for the pixel shift control unit and the display timing control for the display timing control unit when the image is a dynamic image.