(Problem)To provide a novel polyether-modified organopolysiloxane and a novel diorganopolysiloxane-polyether block copolymer that are more resistant to oxidation than the heretofore existing polyoxyalkylene-modified organopolysiloxanes and that are thus more resistant to producing allergenically antigenic oxidation products during elapsed time in storage. Also, to provide methods of producing this novel polyether-modified organopolysiloxane and novel diorganopolysiloxane-polyether block copolymer.(Solution)Organopolysiloxane that is modified by silicon-bonded polyglycidyl ether or glycidyl ether/alkylene oxide copolymer diorganopolysiloxane-polyglycidyl ether block copolymer a method of producing the preceding by a hydrosilylation reaction between organohydrogenpolysiloxane and polyglycidyl ether having a terminal double bond or a glycidyl ether/alkylene oxide copolymer having a terminal double bond and a cosmetic that contains this modified organopolysiloxane or diorganopolysiloxane-polyglycidyl ether block copolymer.
