PURPOSE: To eliminate directionality, to rigidly secure a concrete surface, and to be easily applied to a repair of existing a sewage channel by fusion-bonding or bonding an uneven net at recesses to a sheet of synthetic resin having a plenty of corrosion resistance.

CONSTITUTION: A corrosionproof sheet 1 is composed of a sheet 2 of synthetic resin, and an uneven net 3 secured thereto. The sheet 2 is desirably made of synthetic resin having a plenty of antiacid and antialkaline properties such as polyethylene. The net 3 is made, for example, of a net material knitted with yarns or wire materials of synthetic resin. In order to manufacture the corrosionproof sheet 1, a heated net material is passed between a pair of rolls 5 having uneven faces to be engaged with each other to be formed in a wave shape, the obtained net 3 is contact bonded to the sheet 2 extruded by an extruder 8 between a second roll 5b and a back roll 12, and troughs are thermally fusion-bonded to the sheet 2. The next 3 may be bonded to the sheet 3 with adhesive.