A high-temperature generator having a high durability and reliability is disclosed. The generator comprises a double shell type furnace body defining a combustion chamber and a liquid chamber surrounding the combustion chamber. A burner is arranged at a front end of the combustion chamber for generating high temperature gas within the combustion chamber. A smoke tube assembly including a plurality of smoke tubes is arranged in the liquid chamber aligned along a longitudinal direction of the furnace body. The rear end of the smoke tube opens to the combustion chamber. A rear wall having a double-wall configuration is provided with an aperture arranged to face the rear end of the smoke tube assembly, and the aperture is covered with a cover plate detachably mounted to the rear wall.