The present invention relates to a method for complex processing of apatite concentrate resulting in producing concentrate of rare earth metals (REM) and plaster from phosphogypsum, a waste of sulphuric acid technology for producing phosphoric acid from apatite. The method comprises recrystallization of hemihydrate or anhydrite of calcium sulphate into dihydrate of calcium sulphate in the presence of a soluble calcium salt at concentrations of 0.075-3.75 M (in terms of Ca2+) at subacidic conditions (pH>1), and dissolution with strong acid (pKa<0) at a concentration of 0.2-8.0 M (in terms of H+). Recovery of REM into solution is up to 95%, the residual content of impurities of phosphorus, fluorine and alkali metals in dihydrate of calcium sulphate does not exceed 0.3 wt.%, 0.1 wt.%, 0.05 wt.%, respectively.