Provided is a metallosiloxane compound (A) prepared by reacting a bifunctional silane compound (S1), a monofunctional silane compound (S2), a boron compound (M), and optionally H2O in a molar ratio of [the silane compound (S1)]:[the silane compound (S2)]:[the boron compound (M)]:[H2O] of n:m:k:a, where n, m, k, and a satisfy all conditions (i), (ii), and (iii), in which the metallosiloxane compound has at least one Si—H bond or C2-10 alkenyl group per molecule, and the conditions (i), (ii), and (iii) are expressed as follows: (i) n>0, m>0, k>0, a≧0; (ii) m/n≧0.5; and (iii) (n+m)/k≧1.8.