PURPOSE:To improve the accuracy of the extracted result by putting the powder subjected to grinding after curing under the condition similar to the condition in the stage of molding into a hermetic extracting vessel having an inside wall formed of a fluororesin and wetting the powder with a hydrophilic org. solvent then adding water thereto, heating the powder after stirring and extracting water soluble impurities. CONSTITUTION:An uncured thermosetting resin is cured under the condition similar to a molding condition and thereafter the ground cured powder is put into a hermetic stainless steel vessel 2 of which the whole of the inside wall 1 is made of a fluororesin. After the powder is made wet with a hydrophilic org. solvent, water is added thereto and a piece of rotor is put therein. A stainless steel cover 3 of the wall 1 is fitted, fixed and sealed to the vessel and after stirring, the powder is heated and insoluble impurities are extracted. The insoluble impurities are thus exactly extracted with good accuracy. The contamination from the extracting machine is prevented by using the fluororesin to constitute the whole of the inside wall of the extracting machine.
