Disclosed is a curable composition including an organic polymer containing a crosslinkable silyl group, an ultraviolet ray absorbing agent having a triazine skeleton and a hindered amine based light stabilizer. The composition exhibits improved weather resistance in a thin layer and thick layer portion. Further, disclosed is a curable composition including an organic polymer containing a crosslinkable silyl group, a (meth)acrylic polymer containing an epoxy group, a divalent tin organic carboxylate and an organic amine compound. The composition exhibits excellent properties. Also, disclosed is a curable composition and sealing material including a reactive organic polymer containing at least one crosslinkable silyl group in one molecule thereof, a reactive organic polymer containing less than one crosslinkable silyl group in one molecule thereof, and thermally expandable hollow spheres. The composition has good physical properties. Also, disclosed is a fire-resistant structure forming method.