To separate a high molecular weight substance from a low molecular weight substance with a reduced amount of a solvent employed and in a reduced treatment time compared with a re-precipitation method by washing with an aliphatic hydrocarbon solvent a polysilane having an aromatic group as a substituent.

A polysilane bearing as a substituent an aromatic group, composed of at least one selected from the polysilane skeletons represented by formulae I and II (wherein R is H, an alkyl, aryl, alkoxy, amino or silyl group; R1 is H, an alkyl, aryl, alkoxy or amino group; (x) and (y) are each an integer not less than 0; and x+y=2-10,000), is admixed with 2- to 10-fold amount of an aliphatic hydrocarbon solvent and is washed with stirring for 30 min to 5 hr at a temperature ranging from 10°C to the reflux temperature of the solvent. After washing with agitation, a high molecular weight polysilane having a polymerization degree of at least 20 is recovered as a precipitate and a low molecular weight polysilane having a polymerization degree of less than 20 is recovered by distilling the solvent out of the solution portion. The distilled solvent can be used repeatedly as it is.