A manufacturing apparatus for manufacturing a projection optical system has a plurality of optical modulators, a color-combining optical device and a projection optical device. The manufacturing apparatus positions each of the optical modulators relative to the color-combining optical device and at a back-focus position of the projection optical device. The back-focus position is determined by calculating a deviation amount of the optical path length of a light beam from the optical modulator to the projection optical device based on the external dimensions of the color-combining optical device when the color-combining optical device is interposed between the optical modulator and the projection optical device.
