PURPOSE: To enhance reliability of cutting off water without drop or deviation of bentonite from a resin sheet, and to facilitate a transportation, handling.

CONSTITUTION: A waterproof sheet 1 is formed of two resin sheets 2, 3 and bentonite 4 interposed to be held between the two sheets 2 and 3 entirely in a sandwich structure. One 2 or 3 of the two sheets 2, 3 is formed of a softer sheet than the other sheet 3 in order to easily wind it in a rolled state, and melting point difference of the sheets 2, 3 is set to 25°C or lower. Thus, since the melting point difference of the sheets 2, 3 is small, they can be easily fusion- bonded by a fusion-bonding unit having a heating plate, an impulse sealer, a nichrome wire, etc.