To obtain a heat exchanger capable of continuously monitoring an air mixing state.

A steam supply pipe 3 is connected to a jacket part 2 through an on-off valve 7. A pressure sensor 11 and a temperature sensor 19 are installed in the jacket part 2. A pipe line 8 is mounted at a lower end of the jacket part 2, and is connected to a suction means 6 through an on-off valve 18, a steam trap 4, and a valve 9. The suction means 6 is constituted of a liquid ejector 13, a cooling water tank 14, and a circulation pump 15. A temperature distribution of the outer surface of the jacket part 2 is measured by a temperature distribution measuring instrument 22. Using the temperature distribution measuring instrument 22, it is possible to continuously monitor the presence or absence of air mixing in the jacket part 2.