To provide a water-expansible sealant that has improved storage stability and more excellent performance of mechanical strength, water cutoff performance, etc., after curing than before even when mixed by taking use, etc., in a low-temperature area and environment into consideration.

The water-expansible sealant includes (A) a water-expansible urethane prepolymer, (B) a non-water-expansible urethane prepolymer, (C) an isocyanate compound, (D) a moisture-curing catalyst for urethane and (E) a filler as essential components.