Abstract: One-pack type heat pre-curable moisture-curing sealant composition which comprises as the main component(A) a urethane prepolymer having a content of an active free isocyanate group (NCO) of 0.5 to 13% by weight which is prepared by reacting a polyol and an excess amount of an aromatic polyisocyanate compound, and(B) a vinyl polymer containing a siloxy group, and as an additive(C) a filler in an amount of 20 to 50% by weight based on the whole weight of the sealant, and(D) a plasticizer in an amount of 5 to 30% by weight based on the whole weight of the sealant, which can be precured at a comparatively lower temperature within a short period of time and cured with moisture in air and is useful for adhering various parts in automobile and construction industries without a temporary holding means.