To obtain a heat exchanger capable of promptly increasing heating temperature.

A steam supply pipe 3 is connected to a jacket part 2 via a control valve 7. The steam supply pipe 3 is branched and a preheating fluid tank 12 is connected to the steam supply pipe 3. The outlet side of the preheating fluid tank 12 is connected to the jacket part 2. A suction means 6 is connected to the lower end of the jacket part 2 via a steam trap 4 and an on-off valve 9. The suction means 6 comprises a liquid ejector 13, a cooling water tank 14 and a circulating pump 15. Heating steam having predetermined temperature is stored beforehand in the preheating fluid tank 12. When the heating temperature is changed, by supplying the steam having the predetermined temperature from the preheating fluid tank 12 to the jacket part 2, the heating temperature can be changed promptly.