To provide a decompressed steam heating device, capable of optionally controlling the temperature of a liquid in a tank only by supplying a small quantity of a cooling fluid.

A steam supply pipe 3 is connected to a jacket portion 11 of a reaction vessel 1. A steam pressure regulating valve 7 and a gas-liquid separator 4 are attached to the steam supply pipe 3. A suction unit 2 is connected to a lower portion of the jacket portion 11. The suction unit 2 is composed of an ejector 14, a tank 5, and a circulating pump 15. A circulating liquid outlet port 27 for ejecting a circulating liquid in a shower-like state is arranged above the tank 5. A part of the heat possessed by the circulating liquid is released to the atmosphere while the circulating liquid is ejected in the shower-like state through the circulating liquid outlet port 27, whereby the liquid temperature of the circulating liquid can be reduced.