To obtain a one-pack foamable hardening urethane based material for stopping water, which does not require the use of a catalyst, a foaming accelerator, and the like, together therewith and, when brought into contact with water, foams and hardens to give a foam exhibiting a good water stopping effect.

A mixture of an aromatic hydrophilic polyol (a) and a polyether polyol (b) at a weight ratio of (4:6) to (1:9) is reacted with a polycyclic polyisocyanate to give a prepolymer having an NCO content of 10-25 wt.%. The prepolymer is blended with a silicone based stabilizer and a carbodiimide- modified MDI to give the one-pack foamable hardening urethane based material for stopping water.