PURPOSE: To prevent corrosion of a cooling tube without excessively reducing the cooling surface area of the entire part of the condenser by coating cooling tubes with a corrosion-resisting material.

CONSTITUTION: The low melting point medium of the low melting point medium condenser (for example, fron or the like) which is decomposed when heated at high temperatures, and chlorine is separated from the medium and is combined with hydrogen in the moisture contained in air leaking into the condenser and converted into hydrochloric acid. The hydrochloric acid adheres to the surfaces of the cooling tubes and corrodes the surface of the cooling tubes. That is, since air containing moisture which is not condensed in the condenser is collected in an air cooling tube nest, and extracted through an air extracting tube 22, a large amount of moisture is contained in air in the vicinity of the cooling tubes in the air cooling tube nest. Hence, hydrochloric acid tends to be produced, and the cooling tube in the air cooling tube nest is liable to be corroded in its entirety. Accordingly, the entire cooling tube 10 in the air cooling tube nest 21 is flattened and is coated with a corrosion resisting coating material (such as Teflon) so as to prevent corrosion.