A combined illumination and ventilation duct is disclosed. The present invention includes at least two optical pipes having air flown inside wherein light proceeds within the at least two optical pipes by being totally reflected and a connector connecting the at least two optical pipes together, the connector having a connecting nipple to be connected to a pipe. An upper part of the duct is embedded in a fake ceiling under a real ceiling and a lower part is exposed under the fake ceiling. Alternatively, the whole combined illumination and ventilation duct is installed to be exposed under the fake ceiling. As air and light pass through the combined illumination and ventilation duct, the light is totally reflected within the optical pipes to illuminate an indoor space and the air is supplied or discharged to ventilate the indoor space.