To provide an air heating device for preventing retention of condensate in a heat exchanger.

A steam supply pipe 21 is connected to an inlet header 26 of the heat exchanger 20. A negative pressure/positive pressure selector valve 23 is connected to an outlet header 27. A positive pressure fluid outlet 5 of the negative pressure/positive pressure selector valve 23 is connected to a steam trap 24, and a negative pressure fluid outlet 4 is connected to a condensate recovering means 25. When the inside of the heat exchanger 20 becomes into a negative pressure state not higher than atmospheric pressure, the condensate flows down from the negative pressure fluid outlet 4 to the condensate recovering means 25, is discharged to the outside, and hence does not retain inside.