To provide a crystallization method capable of enhancing crystallization capacity without requiring large electric power.

A reaction part 3 demarcated into an aeration part 3A for aerating and a circulating part 3B not aerating by water-dipped first partition plates 5, and a treating water separation part 4 located at an upper part of the reaction part 3 are provided in a reaction tank 2. The treating water separation part 4 is demarcated into a gas-liquid separation part 4A and a solid-liquid separation part 4B by second partition plates 6 provided above the first partition plates 5 with a space therefrom. An aeration pipe 7 is provided at a lower part of the aeration part 3A, an upward flow of wastewater is formed in the aeration part 3A by opening a wastewater introducing pipe 10. Seed crystal is circulatingly fluidized in the reaction part 3, a chemical agent is added thereto, a part of the circulation flow of the seed crystal in the reaction part 3 is made to flow into the treating water separation part 4, and cleaned treated water by solid-liquid separation in the treating water separation part 4 is discharged from an upper part of the solid-liquid separation part 4B to the outside of the reactor tank 2.