To obtain a resin having excellent weather resistance by adding a silicone block copolymer represented by a specified formula to the system during or after the production of the resin by polymerization.

Dimethallylpolyether, a solvent (e.g. toluene) and a platinum catalyst are fed into a reactor, and dihydropolydimethylsiloxane is added to the mixture. The resulting mixture is reacted at about 80 to 110°C to obtain a linear polysiloxane/polyoxyalkylene block copolymer. This copolymer is dissolved in toluene and polyestermodified polysiloxane and a catalyst are added to the solution. The entire mixture is reacted to obtain a silicone block copolymer represented by formula II [wherein R is a monovalent hydrocarbon; X1 and X2 are each -R1-Z (wherein R1 is a direct bond or a 1-20 C divalent hydrocarbon group; and Z is a reactive functional group); m is 2 or 4; a ≥2; b is in the range: 0≤b≤1; c≤4; d≤1; and Y is a divalent organic group or the like](e.g. a copolymer represented by formula I). This copolymer is mixed with a resin during or after the production by polymerization of the resin to obtain a resin composition.