To efficiently carry out crystallization by performing the intake of water to be treated, crystallization, solid-liquid separation and the discharge of treated water in one apparatus which a treated-water discharge means is provided outside a large cylinder connected to the top of a reaction column, a medium cylinder inside the large cylinder, and a small cylinder having a circulating means inside the medium cylinder.

A reaction column 4 is provided with a means 1 for introducing water to be treated, a means 2 for charging chemicals and a means 3 for discharging crystallized materials. A small cylinder 8 located at the top of the reaction column 4 and a medium cylinder 7 located outside the small cylinder 8 are provided inside a large cylinder 5. A separation part 12 for separating action is formed between the small cylinder 8 and the medium cylinder 7, and a treating part 13 for clarifying action is formed between the medium cylinder 7 and the large cylinder 5. A means 6 for discharging treated water to the outside is provided outside the large cylinder 5. The small cylinder 8 has an equal width from the top to the bottom, and a means 9 for circulating the water to be treated is installed inside the small cylinder 8 so as to accelerate a crystallization reaction. As the means 9 for circulating the water to be treated, specifically, an air diffusion pipe, a circulating pump, an agitator or the like is used.