PURPOSE: To obtain a clay conditioner which prevents muddying, shortens the time required for use after a rain, eliminates the need for rolling compaction and prevents occurrence of frost and weeds, by adding a silane monomer to a porous particle and then heat-treating the silane monomer.

CONSTITUTION: A silica-contg. porous sand particle having a particle diameter of about 0.1W10mm, such as en-tout-cas, crushed stone sand, sea sand, river sand, or volcanic sand, is immersed in a silane monomer solution such as a penetration waterproofing agent of the silane monomer type which is a kind of a reactive water-repellant for construction. If necessary, the sand particle may be previously heated at 60W80°C and may be kneaded in the presence of the silane monomer soln. through application of a pressure by means of a propeller. Thus, the silane monomer is penetrated into the inner surface of the pore. The particle is heat-treated at room temp. to 50°C, thereby obtaining a clay conditioner comprising a sand particle having a silica-contg. repellent surface formed on both the outer surface and the inner surface of the pore.