It is intended to obtain organopolysiloxane capable of forming a cured product having a good balance among properties, i.e., hardness, gas barrier, thermal yellowing resistance, light resistance, heat and cold shock resistance, and adhesion to a base material. The present invention provides organopolysiloxane comprising one or more unsaturated bond-containing group(s) in one molecule and having constitutional units F1, M1, and T in any combination of (i) F1 and M1, (ii) F1 and T, and (iii) F1, M1, and T: F1: M1: T : €ƒ€ƒ€ƒ€ƒ€ƒ€ƒ€ƒ€ƒ (R 1 SiO 3/2 ) c €ƒ€ƒ€ƒ€ƒ€ƒ(3) wherein R 1 represents any selected from the group consisting of a substituted or unsubstituted alkyl group having 1 to 10 carbon atoms, a substituted or unsubstituted cycloalkyl group having 3 to 10 carbon atoms, a substituted or unsubstituted aryl group, and a substituted or unsubstituted aralkyl group; R 2 represents an unsaturated bond-containing group having 2 to 10 carbon atoms; X represents a divalent hydrocarbon group having 2 to 10 carbon atoms; Y represents a divalent hydrocarbon group having 2 to 10 carbon atoms; and a, b, and c each independently represent an integer of 1 or larger, wherein F1 represents a unit constituting cyclic organopolysiloxane.