PURPOSE: To produce products of high purity by providing a means for reducing pressure of a device and/or a means for pressurizing separated liquids containing impurities to obtain a sufficient ratio of circulation rates of separated liquids containing impurities to flow rates of crystal flowing upwardly through a cylinder.

CONSTITUTION: Rotary shafts 3, 3' rotate in opposite directions and screw vanes 4, 4' are provided to transfer upwardly solid matters in a crystal purification cylinder 1, on which cylinder 1 a coarse crystal feed port 6 and a purified crystal takeout port 7 are provided. Chambers 15-1 - 15-n arranged on a side of the cylinder 1, where vanes 4, 4' come into engagement with each other, for taking out separated liquids containing impurities from solid-liquid separation plates 14-1 - 14-n having small holes or comprising netlike plates and a chamber 15 disposed near the lower end of the cylinder 1 for taking out said liquids from a solid-liquid separation plate 5 having small holes or comprising a netlike plate are provided, or, only said chambers 15-1 - 15-n or said chamber 15 are provided, while a pressure reduction line 12 is connected to these chambers. As a result, products of high purity can be produced.