A tunnel kiln consisting of a succession of tunnel sections assembled from prefabricated large-surface side panels and ceiling panels supported on a tunnel frame of structural steel, at least some of the panels being removable for access to the kiln. Similar panels mounted on railcars form the bottom portion of the kiln. The panels are composed of multiple layers of different materials, the material and thickness of the surface layers being adapted to different heat resistance requirements, while the overall dimensions of the composite panels remain the same.