To obtain an anaerobically curable sealant compsn. which has a high flexibility even after subjected to heat hysteresis by compounding a urethane (meth)acrylate prepolymer, a free-radical-polymerizable monomer, an org. peroxide, and fine core-shell particles.

This compsn. comprises a urethane (meth)acrylate prepolymer having a urethane main chain and terminal (meth)acryloyloxy groups, a free- radical-polymerizable monomer [e.g. ethyl (meth)acrylate], an org. peroxide (e.g. cumene hydroperoxide), and 0.1-80wt.% (based on the compsn.) fine core- shell particles. The compsn. may further contain a cure accelerator, a filler, a plasticizer, a tackifier, etc., if necessary. The particles are produced by polymerizing a polymerizable monomer for cores to form cores comprising a rubberlike polymer having a glass transition point lower than the ambient temp. and then polymerizing a polymerizable monomer for shells to form shells comprising a glassy polymer having a glass transition point higher than the ambient temp.