A curing composition comprising (A) a silane compound having formula: R.sup.1.sub.2Si(OR.sup.2).sub.2 wherein R.sup.1 is C.sub.1 C.sub.3 alkyl, C.sub.2 C.sub.3 alkenyl or phenyl, and R.sup.2 is C.sub.1 C.sub.3 alkyl, C.sub.2 C.sub.3 acyl or C.sub.3 C.sub.5 alkoxyalkyl and (B) phosphoric acid is added to an organosilicon compound to formulate a silicone-base coating composition which remains shelf stable and quickly cures at room temperature after being applied to a substrate, to form a cured coating having transparency, surface hardness, adhesion and the like. A silicone-base coating composition having the curing composition formulated therein is effective as a solventless, RTV silicone coating composition of one part type.