To obtain a high-purity organopolysiloxane by adding an alcohol to the reaction mixture so that the concn. of an organopolysiloxane is made to be a specified value and removing a high-molecular compd. produced as a by-product by alcohol extraction.

A polyorganosiloxane of formula IV having a free-radical-polymerizable functional group, a number average mol.wt. of 500-50,000, and a dispersity of 1. 8 or lower is obtd. by subjecting a cyclic compd. of the formula: (R4R5SiO)p (wherein (p) is 3 or 4) (e.g. hexamethylcyclotrisiloxane) to ring-opening polymn. in a solvent in the presence of an initiator of the formula: R1(R2R3SiO)nLi (wherein (n) is 0-500) (e.g. Li trimethylsilanolate), stopping the polymn. by the addition of at least one compd. of formula I, II, or III, removing the solvent by distillation from the polymn. mixture, adding 2-propanol or the like to the mixture to adjust the concn. to 33-50 wt.%, and separating and removing a high-molecular compd. produced as a by-product. In the formulas, R1 to R3 are each a 1-4C alkyl or the like; R4 and R5 are each methyl, phenyl, or the like; R6 is a group represented by formula V, VI, or VII; and X is a halogen or the like.