To obtain a reduced pressure steam heating apparatus capable of uniformly heating an entire heat exchanger.

A steam supply pipe 3 is connected to a jacket part 11 of a reaction furnace 1. A steam pressure control valve 7 is attached to the steam supply pipe 3. A vacuum suction means 2 is communicated with a lower part of the jacket part 11. The vacuum suction means comprises an ejector 14, a tank 5, and a circulation pump 15. A steam discharge port 4 and a temperature-responsive valve 6 are disposed on the right side of the jacket part 11, and a lower end of the jacket part 11 is connected to the ejector 14. When required steam cannot be supplied to the right side part of the jacket part 11, by opening the temperature-responsive valve 6, flow of steam can be forcibly created inside the jacket part 11, and steam can be supplied uniformly to the entire inner side the jacket part 11.