present invention the amount of a monochlorosilane used which is represented by general formula i is appropriately 2 to 20 times by equivalent as much as that of the hexamethylcyclotrisiloxane used from the standpoints of yield and economical efficiency it is particularly desirable that the amount be 3 to 10 times by equivalent a quaternary ammonium salt used in the present invention which is represented by the foregoing formula ii functions as catalyst in the ring opening reaction specific examples of such a quaternary ammonium salt include bu 4 n cl bu 4 n br c 8 h 17 3 men cl and the like of these salts c 8 h 17 3 men cl trioctylmethylammonium chloride is preferred over the others in those formulae bu stands for a butyl group the proportion of the quantity of the quaternary ammonium salt used to the total weight of the monochlorosilane and the hexamethylcyclotri siloxane used which is abbreviated as 34 the weight of silanes 34 hereinafter is properly in the range of 0 01 to 5 by weight from the economical point of view however it is desirable that the proportion be in the range of 0 3 to 2 by weight on the other hand the proportion of the quantity of water used for the hydrolysis to the weight of silanes used is appropriately in the range of 30 to 300 by weight from the standpoints of yield and.