To provide a waterstop agent composition forming an elastic gel form excellent in durability, compression restoring properties, reswelling characteristics during the immersion in water after dried, dimensional and weight stability when allowed to stand still in air and impact resistance by improving fundamentally weak points of conventional waterstop agent comprising polyethylene glycol acrylate; and a method for stopping leaked water.

This waterstop agent composition comprises either (A) 85 to 98 parts by mass of PEG mono(meth)acrylate and (B) 2 to 15 parts by mass of PEG di(meth)acrylate, or (A) 20 to 45 parts by mass of PEG mono(meth)acrylate, (B) 1 to 3 parts by mass of PEG di(meth)acrylate and (C) 50 to 80 parts by mass of a multivalent metal salt containing a (meth)acryloyl group, wherein the composition may further comprise (D) glycols. The present invention provides also a method for stopping leaked water using the waterstop agent composition.