In an air heater for use in recovering, by heat exchange, the heat of an exhaust gas generated from an incinerator for disposing of wastes, prolonged service life and improved working efficiency are attainable with least thermal deformation. The air heater has a heat transfer conduit which is placed in high temperature highly corrosive gas. The transfer conduit comprises a short seal pipe (212) going through a ceiling of said air heater and fixed at the ceiling, a metal ring member (214) arranged outside of the shot seal pipe through a sealing material and arranged to be able to move along a longitudinal axis of said shot seal pipe, a heat transfer pipe (216) formed of a refractory material having a heat conductivity, connected to said metal ring member at a top end portion and sealed at a lower end portion, and a metallic pipe (218) arranged inside of said short seal pipe and said heat transfer pipe in coaxial, providing a through hole at a lower part thereof and providing a metallic support fitting welded at the bottom thereof. The short seal pipe is connected to an inlet pipe of an air to be heated and supported by a structural member of the air heater at a top end thereof. The metallic pipe is connected to an outlet pipe of a heated air and supported by a structural member of the air heater at a top end thereof. The heat transfer pipe is supported by the metallic support fitting by means of being set into a hollow part formed on an inner surface of the heat transfer pipe.