More specifically, hydroxyl groups (--OH) on the surface of a hydrated metal compound and silanol groups (???Si--OH) of a silane-grafted polymer cause a hydrolytic condensation reaction to form strong silanol linkages [--Si--O--Mm On (M is a metal)], under the influences of (a) a small amount of water present on the surface of the hydrated metal compound and (b) the hydrated metal compound which a