Some of the heat mirrors, which have high transmittance in the visible region and high reflectance in the infrared, consist of three thin titanium compound stacked films, TiO2/TiN/ TiO2 on glass substrate. TiO2 is known to have excellent photo-catalytic properties and the top layer may be expected to work as a photo-catalyst as well as the anti-reflection coating. However, strong thickness dependence of the photo-catalytic properties has been reported and the optimum thickness of the top layer for the heat mirror was too thin for the photo-catalytic properties for decomposition of acetaldehyde gas. In this report, we show a possibility to realize a multi-functional heat mirror which shows the photo-catalytic and heat mirror effects simultaneously by optical calculation and preliminary experiments. We found top TiO2 thickness of 350-400nm or about 1170nm showed high heat mirror performance. This thickness was enough to show the photo-catalytic properties. (C) 2004 Elsevier B.V. All rights reserved.