To provide a filler and a water cut-off method by which a gap formed during or after the filling work can be closed up.

The filler 10 comprises a hardening material 12 composed of water, cement and bentonite compounded to have fluidity, and a water expansive material 11 mixed thereto. After the filler 10 is injected in a space which is in contact with water, and after the hardening material 12 is hardened, the filler 10 is dried and shrunk and a crack 13 is formed. The water expansive material 11 exposed on a surface of the crack 13 absorbs the entering water and expands to close up the formed crack 13.