A fuel mixture heating device of an internal combustion engine having a downdraft type carburetor mounted on the collecting portion of the intake manifold. A hollow cylindrical heater vessel is arranged at the lower end of the air horn of the carburetor. The heater vessel comprises an inner pipe, an outer pipe and PTC elements inserted between the inner pipe and the outer pipe for heating the inner pipe before the completion of warm-up of the engine. The inner pipe has a thin wall having a corrugated cross-section. The inner wall of the PTC elements are in contact with the outer wall of the inner pipe. An air gap is present between the outer pipe and the outer walls of the PTC elements. A plurality of plate springs is inserted between the outer pipe and the corresponding PTC elements.