Three decades have passed since the first application of the wet method of deep mixing. This paper presents results of a research on the long-term prperties of cement treated soil twenty years after construction. The long-term properties have two aspects. One is to confirm long-term strength of the cement treated soil. The other is to investigate the deterioration at the boundary surface of the treated soil. The results are that the present strength inside the treated soil mass is 2.1 times greater than 20 years ago, while at the periphery the strength reduction is observed up to a depth of 30 to 50mm from the boundary surface of treated soil.