A novel carbosiloxane dendrimer is disclosed containing a radically polymerizable group in the molecule and which possesses excellent polymerization reactivity. The carbosiloxane dendrimer can be used to provide organic polymers as afforded by the polymerization of the carbosiloxane dendrimer either alone or with other organic monomers. The radically polymerizable group-functional carbosiloxane dendrimer with the general formula (R1 is C1 to C10 alkyl or aryl, R2 is a divalent organic group excluding C1 to C10 alkylene, b is 1 to 3, X1 is a silylalkyl group, and Y is a radically polymerizable group). Also, dendrimer-containing organic polymer as afforded by the polymerization of (A) the aforesaid carbosiloxane dendrimer and (B) radically polymerizable organic monomer.