Provided is a curable composition comprising a reactive silicon group-containing polyoxyalkylene polymer (A), such as a reactive silicon group-containing polyoxypropylene; a reactive silicon group-containing vinyl polymer (B), such as a reactive silicon group-containing (meth)acrylate polymer; and a polyoxyalkylene plasticizer (C) having a low molecular weight, such as polyoxypropylene, the curable composition providing a rubber-like cured object by crosslinking reaction in the presence of moisture. Also provided is a method for sealing ceramic siding boards comprising applying the curable composition as a sealant and curing the curable composition.