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

A reaction tank 2 is demarcated into a reaction part 4 with lower parts communicated with each other and a solid-liquid separation part 5 by a partitioning plate 3 protruded above the water level, the reaction part 4 is demarcated into an aeration part 4A for aeration and a circulation part 4B not aerating by a water-dipped partition plate 6, an aeration tube 7 is set at the lower part of the aeration part 4A, an upward flow of wastewater is formed in the aeration part 4A by opening a wastewater introducing pipe 10, and the bottom of the reaction part 4 is formed into a tilted face 2a tilted diagonally downward from the circulation part 4B toward the aeration part 4A. Seed crystal is circulatingly fluidized in the reaction part 4, a chemical agent is added thereto, a part of the circulation flow of the seed crystal in the reaction part 4 is made to flow into the solid-liquid separation part 5, and cleaned treated water by solid-liquid separation in the solid-liquid separation part 5 is discharged from a treated water discharge pipe 11 to the outside of the reactor tank 2.