The optimal restoration agent for the brittle walls of Idout's tomb built 4400 years ago in Saqqara, Egypt, which is located underground in a desert area, was investigated. From x‐ray diffraction analysis of the sand collected near the tomb, the main ingredient is silica. Alkoxysilane and its oligomer are used because its alkoxy groups can react readily with silica. The compressive test of solidified model sand its main component is silica was done. The increase of both molecular weight of oligomer and the content of oligomer with higher molecular weight effectively developed the compressive force. When an earthquake occurs, it is desirable that the reinforced wall will be crushed in ductility. The addition of di‐alkoxy type alkoxysilane to oligomers changed the failure mode from brittle to ductile, because the network becomes looser. When a moderate quantity of di‐alkoxy type is added to oligomers, the solidified model sand deforms with greater displacement, while maintaining a high compressive force. From scanning electron microscopic observation of fractured solidified specimen, alkoxysilane was making the sand particles connect as a binder. Many cracks were clearly observed in the binder layer. Whereas, the formed crack was far smaller for di‐alkoxy added system.