Disclosed are a novel lactone-modified diketene (meth)acrylate compound having an active methylene group in the molecule and a process for the preparation thereof.The novel lactone-modified diketene(meth)acrylate compound of the present invention can be preferably used as resins for coatings, adhesives, inks, photosensitive materials for photographs, dispersants for pigments, agents for processing fibers, and modifiers for rubbers.Furthermore, disclosed is a novel (co)polymer which comprises the novel lactone-modified diketene(meth)acrylate compound and, optionally, a polymerizable monomer having an ethylenically unsaturated double bond.Still further, disclosed is a curable composition which comprises a lactone-modified diketene(meth)acrylate compound and at least one selected from the group consisting of a melamine, an isocyanate, and aldehyde, and an amine.