Abstract: A silsesquioxane derivative that can further contribute to improving heat conductivity. The silsesquioxane derivative represented in the following formula forms a matrix having exceptional heat conductivity due to curing. [C5] [SiO4/2]s[R1—SiO3/2]t[R2—SiO3/2]u[H—SiO3/2]v[R32—SiO2/2]w[H,R42—SiO1/2]x[R53—SiO1/2]y??(1) [in the formula, R1 is a hydrosilylation-reactive organic group having a carbon-carbon unsaturated bond and having 2 to 30 carbon atoms, R2, R3, R4 and R5 are each independently at least one selected from the group consisting of an alkyl group having 1 to 10 carbon atoms, an aryl group having 5 to 10 carbon atoms and an aralkyl group having 6 to 10 carbon atoms, t, u, w and x are a positive number, and s, v and y are 0 or a positive number].