PURPOSE: To easily improve the mechanical strength, flexibility or the like of a stone product in sheet form used as a building material by packing an impregnating material into the pores, cracks or the like existing in the stone product.

CONSTITUTION: The impregnating material 3, such as water glass or silicate, is previously injected into the stone product 1 in sheet form used indoors and outdoors as the external material, furniture or the like of a building or the like to pack the impregnating material 3 into the cracks, pores or the like existing in the stone product 1. At the time of the injection treatment, foreign matter, such as air, is preferably previously removed from the stone product 1. The injection treatment of the impregnating material 3 may be adequately executed by pressure injection of about 0.8 to 1 MPa and for a better effect, the injection of the impregnating material 3 is carried out under a high pressure of about ≤15 MPa. As a result, the mechanical strength and flexibility of the stone product 1 may be improved and, therefore, the thickness of the stone product may be made thinner.