To prevent soluble earth and sand from being converted into sludge and muddy water as the result of rainfall and flowing out into rivers, lakes or the sea, etc., by improving the soluble earth and sand into insoluble earth and sand differing in nature from conventional improved earth.

Insoluble earth and sand 1 is dumped into a mixing vessel 5 and mixed with an aqueous solution 55 of a cement type improver 51 and a fibrous material 52 to produce primary improved earth 10. The primary improved earth 10 is mixed with an aqueous solution 65 of a polymer based improver 61 and agitated to produce insoluble improved earth 20.