To hardly cause breakage by bonding the face of a sheet formed by layering the paper having a specific tensile strength and organic fiber fabric on each other, to the rear surface of a hydraulic foundation bed.

Flat fabric constituted of polyester fiber, polypropylene fiber or the like, for example, and the paper whose longitudinal tensile strength is 34 kgf or higher are bonded and layered with low density polyethylene or the like and is formed into a sheet shape having a waterproof layer. On a base such as cement or gypsum, a plate-shape hydraulic substrate containing aggregate, reinforcing fiber such as glass fiber, and light-weight material such as perlite is formed. The rear surface of a hydraulic foundation bed and the face of a sheet are bonded to each other to lower elastic modulus near the rear surface by a combined rule, decreasing generated stress against a fixed strain amount to improve flexibility. It is thus possible to prevent breakage at the time of transport or construction, perform construction so as to meet winding bedding, attain thickness and weight reduction, and prevent generation of warp, frost damage and the like by preventing water or moisture absorption from the rear surface.