In a system for separating crystals from sludge, or from separated water generated when sludge is subjected to a concentration process or a dewatering process, aspects of both phosphorus removal and phosphorus recovery can be satisfied, MAP having a high degree of purity can be recovered, and stable treatment can be performed such that the MAP recovery rate is high and blockages of a hydrocyclone do not occur. An aspect of the present invention is a crystal separation apparatus for separating crystals from sludge, or from separated water generated when sludge is subjected to a concentration process or a dewatering process, comprising a hydrocyclone for separating the crystals from the sludge or separated water introduced into the hydrocyclone, and a return pipe A for returning the sludge or separated water which flows out from an overflow riser of the hydrocyclone to a sludge or separated water introduction portion of the hydrocyclone.