To provide a device for crystallizing by low pressure steam, which is able to improve the production efficiency and the quality of a crystal by crystallizing with a large amount of heat of the steam.

A pressure control valve 19 is connected to a steam supplying pipe 15 and a jacket part 6 of a crystallization tank 1 is connected to the steam supplying pipe 15 through a temperature control valve 23. Further, an ejector- type combined pump 16 as a sucking means is connected to the lower part of the jacket part 6 through a communication pipe 25. To the communication pipe 25, a steam trap 27 and a bypath valve 28 are attached. The ejector-type combined pump 16 is constituted by communicating an ejector 26, a tank 31 and a circulation pump 32. The inside of the jacket part 6 is maintained at about atmospheric pressure or a pressure lower than the atmospheric pressure by the ejector-type combined pump 16, and the crystallization tank 1 is heated by steam of about 100°C or of a temperature lower than 100°C.