Abstract: The present invention provides a method for preventing the decrease of the efficiency of heat transfer, caused by frost formation, in a precooler of an air breathing engine. In the precooler, the heat exchanger tube is provided. Into the heat exchanger tube, liquid hydrogen serving as a coolant is supplied. The main air flow is sent to the precooler via the intake duct, and the main air flow which is cooled down there, is then sent to a compressor of the air breathing engine. Via the nozzle provided in the middle of the intake duct, ethanol is mixed into the main air flow. Ethanol is carried into the precooler, and condenses together with the water vapor on the surface of the heat exchanger tube. A frost layer thus formed has less cavities and low thermal resistance as compared to that formed when the water vapor condenses solely. Therefore, the decrease of the performance of the heat exchanger, which is caused by the frost formation, can be prevented.