To provide a crystallization reaction apparatus and a crystallization reaction method capable of efficiently performing crystallization reaction even when the crystal concentration inside a crystallization reactor varies.

A crystallization reaction apparatus includes an agitation device 34 having an agitation blade 38, and includes a crystallization reactor 10 having a crystallization reaction part 28 for generating a crystal of a hardly soluble salt by adding a calcium agent to raw water containing a crystallization target substance, and the agitation device 34 adjusts the rotation speed of the agitation blade 38 according to the crystal concentration inside the crystallization reaction part 28.